null type id handling does not work with writeTypePrefix()

[cowtowncoder]

Describe your Issue

(note: regression partly due to #3373, reported f.ex as #4354 (but not just that))

So: if custom TypeIdResolver returns null from idFromValue() or idFromValueAndType(), result is that underlying JsonGenerator.writeTypePrefix() call is omitted. This may work for some cases, but won't for others, because necessary parts of structure are missing.

Conceptually the idea would be that null for Type Id means "do not add Type Id", but doing so cannot be done by simply skipping calls to JsonGenerators writeTypePrefix() and writeTypeSuffix(): rather, calls need to be made but these methods need to be aware of possibly missing type id and output whatever else is necessary.