Fix warnings for make_motion_gather_to_*().

After 7e268107 started seeing warnings like
------------------
cdbgroup.c:1478:68: warning: expression which evaluates to zero treated as a null pointer constant of type 'List *' (aka 'struct List *') [-Wnon-literal-null-conversion]
                result_plan = (Plan*)make_motion_gather_to_QE(root, result_plan, false);
                                                                                 ^~~~~
------------------

src/backend/cdb/cdbgroup.c

@@ -1486,7 +1486,7 @@ make_two_stage_agg_plan(PlannerInfo *root,
 		break;
 			
 	case MPP_GRP_TYPE_PLAIN_2STAGE:
-		result_plan = (Plan*)make_motion_gather_to_QE(root, result_plan, false);
+		result_plan = (Plan*)make_motion_gather_to_QE(root, result_plan, NULL);
 		result_plan->total_cost += 
 			incremental_motion_cost(result_plan->plan_rows,
 									result_plan->plan_rows * root->config->cdbpath_segments);
@@ -2253,7 +2253,7 @@ make_plan_for_one_dqa(PlannerInfo *root, MppGroupContext *ctx, int dqa_index,
 		Assert(ctx->numGroupCols == 0); /* No grouping columns */
 		Assert(n == 1);
 
-		result_plan = (Plan*)make_motion_gather_to_QE(root, result_plan, false);
+		result_plan = (Plan*)make_motion_gather_to_QE(root, result_plan, NULL);
 		result_plan->total_cost += 
 				incremental_motion_cost(result_plan->plan_rows,
 						result_plan->plan_rows * root->config->cdbpath_segments);

src/backend/cdb/cdbsetop.c

@@ -161,12 +161,12 @@ adjust_setop_arguments(PlannerInfo *root, List *planlist, GpSetOpType setop_type
 				case CdbLocusType_HashedOJ:
 				case CdbLocusType_Strewn:
 					Assert( subplanflow->flotype == FLOW_PARTITIONED );
-					adjusted_plan = (Plan*)make_motion_gather_to_QD(root, subplan, false);
+					adjusted_plan = (Plan*)make_motion_gather_to_QD(root, subplan, NULL);
 					break;
 					
 				case CdbLocusType_SingleQE:
 					Assert( subplanflow->flotype == FLOW_SINGLETON && subplanflow->segindex == 0 );
-					adjusted_plan = (Plan*)make_motion_gather_to_QD(root, subplan, false);
+					adjusted_plan = (Plan*)make_motion_gather_to_QD(root, subplan, NULL);
 					break;
 
 				case CdbLocusType_Entry:
@@ -191,7 +191,7 @@ adjust_setop_arguments(PlannerInfo *root, List *planlist, GpSetOpType setop_type
 				case CdbLocusType_Strewn:
 					Assert( subplanflow->flotype == FLOW_PARTITIONED );
 					/* Gather to QE.  No need to keep ordering. */
-					adjusted_plan = (Plan*)make_motion_gather_to_QE(root, subplan, false);
+					adjusted_plan = (Plan*)make_motion_gather_to_QE(root, subplan, NULL);
 					break;
 					
 				case CdbLocusType_SingleQE: