bugfix: null-bitmap handing in tcache_column_store using bulk-exec mode

tcache_column_store being associated with a particular relation, its
structure follows table definition; that means tcs reflects not-NULL
constrains in the table definition.
Once a tcache_column_store is fetched to upper plan-node using
bulk-exec mode, we have to pay attention TupleDesc of GpuSort.
Even though it has symmetric definition, here is no not-null constratins.

I put a workaround here; that accept tcache_column_store with
cdata[].isnull == NULL. It shall be eventually revised when we support
"simplified" projection on GpuSort also.