Remove JB_ISEMPTY().  Better "raw scalar" handling.

JB_ISEMPTY() defensive checks seem quite unnecessary, except in exactly
one place where an equivalent check now performed instead.

Have "raw scalar" handling occur within worker function called by bt
support function 1.  This seems like a way better level to handle the
issue at, not least since it's an implementation detail already referred
to at numerous other points within jsonb_util.c (and not jsonb_op.c).