Add `AvroGenerator.Feature.ADD_NULL_AS_DEFAULT_VALUE_IN_SCHEMA` for adding default `null` in generated schema

[kucera-jan-cz]

Added:     new Feature enum AVRO_DEFAULT_ENABLED for enabling null inclusion     RecordVisitor is now include NullNode if AVRO_DEFAULT_ENABLED is enabled and field is not primitive

[cowtowncoder]

@kucera-jan-cz Apologies for slow follow up. Hope to review soon now, finally. One request: could this be rebased (or new PR created) for 2.10? master targets 3.0.0, and that's quite a ways away: so it'd be great to get this in 2.10 instead (from which I'll merge to master).

[cowtowncoder]

Ok: so, aside from one comment on Javadoc for the new feature (need to indicate exactly where it applies), I have one other question -- does this check that field type allows nulls? In Avro null is a distinct type so default value of null is only legal for union type that include null type.

[baharclerode]

does this check that field type allows nulls?

Yes-- My comment was on the conditional that makes this check; Specifically, if:

• The user did not specify a default value (Such as with @AvroDefault() or @JsonProperty(defaultValue=))
• Default nulls feature is enabled
• The schema is a UNION and NULL is a valid type in that union

then set the default value to null.

[cowtowncoder]

Hmmh. Ok, I think I'll merge this in and change naming.

Looks like there's one snag wrt 2.10, so will first just merge to master.