fixing bug with read replica outputs when no RR is set

main.tf

@@ -187,13 +187,13 @@ output "rds_instance_address" {
 }
 
 output "read_replica_rds_instance_id" {
-    value = "${aws_db_instance.read_replica.id}"
+    value = "${var.read_replica_count == "0" ? "null" : element(concat(aws_db_instance.read_replica.*.id, list("")), 0) }"
 }
 
 output "read_replica_rds_instance_address" {
-    value = "${aws_db_instance.read_replica.address}"
+    value = "${var.read_replica_count == "0" ? "null" : element(concat(aws_db_instance.read_replica.*.address, list("")), 0) }"
 }
 
 output "subnet_group_id" {
     value = "${aws_db_subnet_group.default.id}"
-}
\ No newline at end of file
+}