Test and fix support for serializing nulls

[davidmorgan]

There are various issues filed around nulls--collecting them here.

[larssn]

Combining #1015 and #1027 fixes #604, #1040 and #1013.

We've been using those two PR's for a bit over a month in production, for what it's worth.

[davidmorgan]

Note to self: there is an issue w/ generated code for an Object? field.

[davidmorgan]

All the fixes have merged--will be in the next release.

[jscti]

@davidmorgan Digging in the closed issues, docs, and BuiltValueField properties but didn't found anything to enable null properties serialization to produce field: null instead of totally removing it.

Is it fixed ?

I'm on 8.1.4

Thanks

[kuhnroyal]

There is still #912 open