Don't bother copying empty support arrays in a zero-column MergeJoin.

The case could not arise when this code was originally written, but it can
now (since we made zero-column MergeJoins work for the benefit of FULL JOIN
ON TRUE).  I don't think there is any actual bug here, but we might as well
treat it consistently with other uses of COPY_POINTER_FIELD().  Per comment
from Ashutosh Bapat.

src/backend/nodes/copyfuncs.c

@@ -672,10 +672,13 @@ _copyMergeJoin(const MergeJoin *from)
 	 */
 	COPY_NODE_FIELD(mergeclauses);
 	numCols = list_length(from->mergeclauses);
-	COPY_POINTER_FIELD(mergeFamilies, numCols * sizeof(Oid));
-	COPY_POINTER_FIELD(mergeCollations, numCols * sizeof(Oid));
-	COPY_POINTER_FIELD(mergeStrategies, numCols * sizeof(int));
-	COPY_POINTER_FIELD(mergeNullsFirst, numCols * sizeof(bool));
+	if (numCols > 0)
+	{
+		COPY_POINTER_FIELD(mergeFamilies, numCols * sizeof(Oid));
+		COPY_POINTER_FIELD(mergeCollations, numCols * sizeof(Oid));
+		COPY_POINTER_FIELD(mergeStrategies, numCols * sizeof(int));
+		COPY_POINTER_FIELD(mergeNullsFirst, numCols * sizeof(bool));
+	}
 
 	return newnode;
 }