Remove a useless PG_GETARG_DATUM() call from jsonb_build_array.

Remove a useless PG_GETARG_DATUM() call from jsonb_build_array.

This loop uselessly fetched the argument after the one it's currently
looking at.  No real harm is done since we couldn't possibly fetch off
the end of memory, but it's confusing to the reader.

Also remove a duplicate (and therefore confusing) PG_ARGISNULL check in
jsonb_build_object.

I happened to notice these things while trolling for missed null-arg
checks earlier today.  Back-patch to 9.5, not because there is any
real bug, but just because 9.5 and HEAD are still in sync in this
file and we might as well keep them so.

In passing, re-pgindent.