Serialize empty tags to actual json array/object

[Croway]

Hello @cowtowncoder , I think it would be nice to serialize empty tag into actual array/objects (in some cases), not text value, in particular, as you can see from the added test case <name><first/><last>[]</last></name> will be serialized as {"first":[],"last":"[]"} when using JsonNode, the empty tag first is serialized as an empty json array, not a text value.

But I feel like that my solution is a bit hacky, do you have any hint?

Once #640 is merged I'll rebase this one.

[cowtowncoder]

Yeah I am not sure I'm inclined to add support for this. Jackson, in general, tries to steer clear of 2 general areas:

1. Validation of logical content (aside from well-formedness and needing to map between data, Java Objects) -- Bean Validation API to be used for validation, constraints-checking
2. Structural transformations

Obviously there's some amount of functionality that overlaps here, but in general complexity of structural transformations is such, for streaming/incremental reading/writing that it seems best left out.
Things that are needed are already too complex.