The original patch to avoid building a hash join's hashtable when the

outer relation is empty did not work, per test case from Patrick Welche.
It tried to use nodeHashjoin.c's high-level mechanisms for fetching an
outer-relation tuple, but that code expected the hash table to be filled
already.  As patched, the code failed in corner cases such as having no
outer-relation tuples for the first hash batch.  Revert and rewrite.