Handle WindowClause.runCondition in tree walker/mutator functions.

Handle WindowClause.runCondition in tree walker/mutator functions.

Commit 9d9c02ccd, which added the notion of a "run condition" for
window functions, neglected to teach nodeFuncs.c to process the new
field.  Remarkably, that doesn't seem to have had any ill effects
before we invented Var.varnullingrels, but now it can cause visible
failures in join-removal scenarios.

I have no faith that there's not reachable problems in v15 too,
so back-patch the code change to v15 where 9d9c02ccd came in.
The test case seems irrelevant to v15, though.

Per bug #18277 from Zuming Jiang.  Diagnosis and patch by
Richard Guo.

Discussion: https://postgr.es/m/18277-089ead83b329a2fd@postgresql.org