[Expression] fix empty string as expression issue

[feich-ms]

fix: #2308

[coveralls]

Pull Request Test Coverage Report for Build 83104

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 78.506%

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Totals
Change from base Build 83089:	1.2%
Covered Lines:	10563
Relevant Lines:	13455

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💛 - Coveralls

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.5.3

[tomlm]