Implement `JsonNodeFeature.WRITE_NULL_PROPERTIES` to allow skipping JSON `null` values on writing

[cowtowncoder]

(note: follow up beyond #3405 and part of JSTEP-7)
(note: similar to #3421)

Let's add:

JsonFeature.WRITE_NULL_PROPERTIES (default: true)

disabling of which allows not writing of NullNode-valued properties.