return default instance on unchanged builder

[SentryMan]

I can't see a reason to return separate instances.

Essentially this change causes the same Jsonb instance to be returned by the statement Jsonb.builder().build()

[rbygrave]

I see this as a performance optimisation, but I don't see it being a very commonly used one because factories is generally not empty, so I see as a not very useful performance optimisation?

What was a motivation for this?

[SentryMan]

For Jsonb, there are a bunch of places where a default jsonb instance can get created. (inject plugin, jex default adapter, http-client default adapter)

I've also got some internal libraries that create default jsonb instances via SPI

[SentryMan]

factories is generally not empty

If there are no manually provided factories, it will be empty by the time the check happens. Otherwise that test I added would fail