fix: Don't define backup_configuration if default DB is configured as a replica (#559)

Co-authored-by: Imran Nayer <[email protected]>

Author: OscarVanL

modules/mysql/main.tf

@@ -70,7 +70,7 @@ resource "google_sql_database_instance" "default" {
     connector_enforcement       = local.connector_enforcement
 
     dynamic "backup_configuration" {
-      for_each = [var.backup_configuration]
+      for_each = var.master_instance_name != null ? [] : [var.backup_configuration]
       content {
         binary_log_enabled             = local.binary_log_enabled
         enabled                        = local.backups_enabled