pg_plan_advice: Avoid assertion failure with partitionwise aggregate.

pg_plan_advice: Avoid assertion failure with partitionwise aggregate.

An Append node that is part of a partitionwise aggregate has no
apprelids. If such a node was elided, the previous coding would
attempt to call unique_nonjoin_rtekind() on a NULL pointer, which
leads to an assertion failure. Insert a NULL check to prevent that.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: http://postgr.es/m/0afba1ce-c946-4131-972d-191d9a1c097c@gmail.com