feat: update CSQL MySQL prebuilt tools to use IAM (#2202)

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🛠️ Fixes #<issue_number_goes_here>

internal/prebuiltconfigs/tools/cloud-sql-mysql.yaml

@@ -19,8 +19,8 @@ sources:
     region: ${CLOUD_SQL_MYSQL_REGION}
     instance: ${CLOUD_SQL_MYSQL_INSTANCE}
     database: ${CLOUD_SQL_MYSQL_DATABASE}
-    user: ${CLOUD_SQL_MYSQL_USER}
-    password: ${CLOUD_SQL_MYSQL_PASSWORD}
+    user: ${CLOUD_SQL_MYSQL_USER:}
+    password: ${CLOUD_SQL_MYSQL_PASSWORD:}
     ipType: ${CLOUD_SQL_MYSQL_IP_TYPE:PUBLIC}
 tools:
   execute_sql: