intarray: return empty zero-dimensional array for an empty array

Previously a one-dimensional empty array was returned, but its text
representation matched a zero-dimensional array, and there is no way to
dump/reload a one-dimensional empty array.

BACKWARD INCOMPATIBILITY

Per report from elein

contrib/intarray/_int_tool.c

@@ -246,6 +246,13 @@ resize_intArrayType(ArrayType *a, int num)
 	int			nbytes = ARR_DATA_OFFSET(a) + sizeof(int) * num;
 	int			i;
 
+	/* if no elements, return a zero-dimensional array */
+	if (num == 0)
+	{
+		ARR_NDIM(a) = 0;
+		return a;
+	}
+
 	if (num == ARRNELEMS(a))
 		return a;