Fix "wrong varnullingrels" for subquery nestloop parameters.

Fix "wrong varnullingrels" for subquery nestloop parameters.

If we apply outer join identity 3 when relation C is a subquery
having lateral references to relation B, then the lateral references
within C continue to bear the original syntactically-correct
varnullingrels marks, but that won't match what is available from
the outer side of the nestloop.  Compensate for that in
process_subquery_nestloop_params().  This is a slightly hacky fix,
but we certainly don't want to re-plan C in toto for each possible
outer join order, so there's not a lot of better alternatives.

Richard Guo and Tom Lane, per report from Markus Winand

Discussion: https://postgr.es/m/DFBB2D25-DE97-49CA-A60E-07C881EA59A7@winand.at