Remove over-optimistic Assert.

In commit 2489d76c4, I'd thought it'd be safe to assert that a
PlaceHolderVar appearing in a scan-level expression has empty
nullingrels.  However this is not so, as when we determine that a
join relation is certainly empty we'll put its targetlist into a
Result-with-constant-false-qual node, and nothing is done to adjust
the nullingrels of the Vars or PHVs therein.  (Arguably, a Result
used in this way isn't really a scan-level node, but it certainly
isn't an upper node either ...)

It's not clear this is worth any close analysis, so let's just
take out the faulty Assert.

Per report from Robins Tharakan.  I added a test case based on
his example, just in case somebody tries to tighten this up.

Discussion: https://postgr.es/m/CAEP4nAz7Enq3+DEthGG7j27DpuwSRZnW0Nh6jtNh75yErQ_nbA@mail.gmail.com

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