postgres_fdw: Fix assertion in estimate_path_cost_size().

postgres_fdw: Fix assertion in estimate_path_cost_size().

Commit 08d2d58a2 added this assertion assuming that the retrieved_rows
estimate for a foreign relation is set to at least one row in
estimate_path_cost_size(), but the assumption isn't correct because if
the relation is a foreign table with tuples=0, the estimate would be set
to 0 in there when using local stats, and if the query's WHERE clause
has a NULL condition, the estimate could be set to 0 in there when using
remote estimates.  (Note: even in the latter case the assertion could be
reachable when costing foreign final paths.)  Repair.

Per bug #17713 from Robins Tharakan.  This patch was already applied to
v13 or later; apply it to v12 as well where the aforementioned commit
was added.  Thanks to Richard Guo for investigation.

Discussion: http://postgr.es/m/17713-92cce66de7e81c04%40postgresql.org
Discussion: http://postgr.es/m/CAEP4nAza%2B0fTCLkgkKYux3JDo3tUBTQORehP%2BaCxSNURpSFpHw%40mail.gmail.com