Compare varnullingrels too in assign_param_for_var().

Compare varnullingrels too in assign_param_for_var().

Oversight in 2489d76c4.  Preliminary analysis suggests that the
problem may be unreachable --- but if we did have instances of
the same column with different varnullingrels, we'd surely need
to treat them as different Params.

Discussion: https://postgr.es/m/412552.1706203379@sss.pgh.pa.us