pgsql: postgres_fdw: Fix assertion in estimate_path_cost_size().

postgres_fdw: Fix assertion in estimate_path_cost_size().

Commit 08d2d58a2 added an assertion assuming that the retrieved_rows
estimate for a foreign relation, which is re-used to cost pre-sorted
foreign paths with local stats, is set to at least one row in
estimate_path_cost_size(), which isn't correct because if the relation
is a foreign table with tuples=0, the estimate would be set to 0 there
when not using remote estimates.

Per bug #16807 from Alexander Lakhin. Back-patch to v13 where the
aforementioned commit went in.

Author: Etsuro Fujita
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/16807-9fe4e08fbaa5c7ce%40postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5e7fa189ee92d5ecf42a295c336625d71bfe876d

Modified Files
--------------
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(-)