Fix incorrect non-strict join recheck in MERGE WHEN NOT MATCHED BY SOURCE.

If a MERGE command contains WHEN NOT MATCHED BY SOURCE actions, the
merge join condition is used by the executor to distinguish MATCHED
from NOT MATCHED BY SOURCE cases. However, this qual is executed using
the output from the join subplan node, which nulls the output from the
source relation in the not matched case, and so the result may be
incorrect if the join condition is "non-strict" -- for example,
something like "src.col IS NOT DISTINCT FROM tgt.col".

Fix this by enhancing the join recheck condition with an additional
"src IS NOT NULL" check, so that it does the right thing when
evaluated using the output from the join subplan.

Noted by Tom Lane while investigating bug #18634 from Alexander
Lakhin.

Back-patch to v17, where WHEN NOT MATCHED BY SOURCE support was added
to MERGE.

Discussion: https://postgr.es/m/18634-db5299c937877f2b%40postgresql.org

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