When targetlist is NULL, ExecTargetList was passing back a

pointer to palloc'd but uninitialized memory.  This is not cool; anyone looking
at the returned 'tuple' would at best coredump and at worst behave in a
bizarre and irreproducible way.  Fix it to return a predictable value,
namely a correctly-set-up palloc'd tuple containing zero attributes.
I believe this fix is both safe and critical.