Fix oversight in my recent patch to allow ExecMakeFunctionResult to handle

Fix oversight in my recent patch to allow ExecMakeFunctionResult to handle
materialize-mode set results.  Since it now uses the ReturnSetInfo node
to hold internal state, we need to be sure to set up the node even when
the immediately called function doesn't return set (but does have a set-valued
argument).  Per report from Anupama Aherrao.