Handle null type id for polymorphic values that use external type id

[stormboy]

Remove exceptions thrown when polymorphic value is null.
If there is a need to force non-null value, this could be provided as an extra property in @JsonTypeInfo, or perhaps use an existing property such as JsonProperty.required.

[cowtowncoder]

What actual problem is this fixing? Could you provide a test case to show the problem you have. I am not sure, looking at the patch alone, that this is a valid change. Type Ids are typically required, unless there is defaultImpl defined. How should actual expected polymorphic type determined, if no type id is available?

[stormboy]

This is a case when there exists a type Id via an EXTERNAL_PROPERTY but the polymorphic value itself is null. See http://stackoverflow.com/questions/28089484/deserialization-with-jsonsubtypes-for-no-value-missing-property-error for an example.

[cowtowncoder]

Hmmh ok. That's bit unusual (nulls are not typed, so there really should not be an id IMO), but I guess it could be supported.

Would it be possible to add a simple unit test, with commentary, both to verify the fix and as bit of documentation?

[cowtowncoder]

@stormboy Thank you for suggesting this, it makes sense. I ended up solving the issue slightly differently, so I won't be merging the patch as is, but the end result should be same. Fix will be in 2.6.3.

[stormboy]

Thanks @cowtowncoder Sorry I didn't get back with a unit test and commentary. I had been planning to, but have been busy working on other things.

[cowtowncoder]

@stormboy No problem -- SO issue and your comments clarified this enough so it was not a big deal in the end. But if you do get a chance, it would be great to verify this against 2.6.3-SNAPSHOT, if you can do a local build.