`JsonFormat.Feature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT` not working with `null` coercion with `@JsonSetter`

[cowtowncoder]

Looks like coercion from null (see #1402) does not yet work if null is result of coercion from empty String, when JsonFormat.Feature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT is enabled.
But it should.

This is particularly important for XML use case where lack of native null (and resulting confusion between empty String / null) can cause impedance.