Tolerant null in LG context

[Danieladu]

close: #3326

Original issue:

# hello
- @{user.name}

Today if user.name is null, it will result an exception, we should change this to "null" (we can optimize the result later) so that user won't be interrupted by exceptions.

Expected result:

>if a == null or a.b == null, result would be string "null"
# template1
- @{a.b} 

> if a== null or a.b == null, result would be "result is 'null'"
# template2
- switch: @{a.b}
- CASE: @{'null'}
- result is 'null'

> if a template ref is return null, the result would be 
>{ "lgType":"MyStruct", "key1":"null"}
# template3
[MyStruct
    key1 = @{template4()}
]

> return null
# template4
- IF:@{false}
- hello 

[coveralls]

Pull Request Test Coverage Report for Build 104114

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 17 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+0.03%) to 78.059%

---

Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs/ObjectPath.cs	1	86.46%
/libraries/Microsoft.Bot.Builder.Dialogs/Prompts/OAuthPrompt.cs	1	56.4%
/libraries/Microsoft.Bot.Builder/Inspection/InspectionMiddleware.cs	1	78.89%
/libraries/Microsoft.Bot.Builder.LanguageGeneration/LGErrors.cs	1	55.56%
/libraries/Microsoft.Bot.Expressions/BuiltInFunctions.cs	1	93.21%
/libraries/Microsoft.Bot.Expressions/LRUCache.cs	1	82.93%
/libraries/Microsoft.Bot.Expressions/TriggerTrees/Trigger.cs	1	85.09%
/libraries/Microsoft.Bot.Builder.Azure/CosmosDbStorageOptions.cs	2	75.0%
/libraries/Microsoft.Bot.Builder/MemoryTranscriptStore.cs	2	86.09%
/libraries/Microsoft.Bot.Builder.LanguageGeneration/Evaluator.cs	6	92.83%

Totals
Change from base Build 104013:	0.03%
Covered Lines:	13530
Relevant Lines:	17333

---

💛 - Coveralls

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.ApplicationInsights.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Testing.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Streaming.dll compared against version 4.6.3

[xieofxie]

will this be a public option instead of a private one?
For example, when developing, report null with error and when in production, replace null with custom warning like: [Not Set] and also trace it in telemetry for further investigation.

[boydc2014]

will this be a public option instead of a private one?
For example, when developing, report null with error and when in production, replace null with custom warning like: [Not Set] and also trace it in telemetry for further investigation.

This PR's intention is to make not throw exception as default. We will explore whether make it configurable with something like "strict=true", which is out of scope of this one, and will be taken into considerations with other possible configurable behavior together.

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.ApplicationInsights.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Testing.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Streaming.dll compared against version 4.6.3

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.ApplicationInsights.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Testing.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Streaming.dll compared against version 4.6.3