Revert "Avoid null pointer deref in partition selection"

Commit 7b6dc9bf was well-intentioned,
but wrong: in cases where `outerPlan` is NULL, inputSlot should be
`NULL`, but the execution of the partition selector should *NOT*
prematurely end there, instead, a static partition selection should
happen (once) before this operator was exhausted (returning NULL).

This is a pure revert commit. The `NULL` initialization will follow.

This reverts commit 7b6dc9bf.

src/backend/executor/nodePartitionSelector.c

@@ -146,7 +146,7 @@ ExecPartitionSelector(PartitionSelectorState *node)
 	PartitionSelector *ps = (PartitionSelector *) node->ps.plan;
 	EState	   *estate = node->ps.state;
 	ExprContext *econtext = node->ps.ps_ExprContext;
-	TupleTableSlot *inputSlot = NULL;
+	TupleTableSlot *inputSlot;
 	TupleTableSlot *candidateOutputSlot;
 
 	if (ps->staticSelection)
@@ -181,12 +181,12 @@ ExecPartitionSelector(PartitionSelectorState *node)
 		PlanState *outerPlan = outerPlanState(node);
 		Assert(outerPlan);
 		inputSlot = ExecProcNode(outerPlan);
-	}
 
-	if (TupIsNull(inputSlot))
-	{
-		/* no more tuples from outerPlan */
-		return NULL;
+		if (TupIsNull(inputSlot))
+		{
+			/* no more tuples from outerPlan */
+			return NULL;
+		}
 	}
 
 	/* partition elimination with the given input tuple */