Fix coredump problem in plpgsql's RETURN NEXT. When a SELECT INTO

Fix coredump problem in plpgsql's RETURN NEXT.  When a SELECT INTO
that's selecting into a RECORD variable returns zero rows, make it
assign an all-nulls row to the RECORD; this is consistent with what
happens when the SELECT INTO target is not a RECORD.  In support of
this, tweak the SPI code so that a valid tuple descriptor is returned
even when a SPI select returns no rows.