pgsql: 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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/69f75bf825f83dc506e73cf88f0adde541c39004

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 18 ++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 2 +-
contrib/postgres_fdw/sql/postgres_fdw.sql | 12 ++++++++++++
3 files changed, 31 insertions(+), 1 deletion(-)