Fix uninitialized-variable bug.

map_partition_varattnos() failed to set its found_whole_row output
parameter if the given expression list was NIL.  This seems to be
a pre-existing bug that chanced to be exposed by commit 6f6b99d1.
It might be unreachable in v10, but I have little faith in that
proposition, so back-patch.

Per buildfarm.