Fix computation of varnullingrels when const-folding field selection.

We can simplify FieldSelect on a whole-row Var into a plain Var
for the selected field.  However, we should copy the whole-row Var's
varnullingrels when we do so, because the new Var is clearly nullable
by exactly the same rels as the original.  Failure to do this led to
errors like "wrong varnullingrels (b) (expected (b 3)) for Var 2/2".

Richard Guo, per bug #18184 from Marian Krucina.  Back-patch to
v16 where varnullingrels was introduced.

Discussion: https://postgr.es/m/18184-5868dd258782058e@postgresql.org

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