Fix exports migration when there is none (#14761)

frigate/util/config.py

@@ -50,6 +50,7 @@ def migrate_frigate_config(config_file: str):
         previous_version = "0.14"
 
         logger.info("Migrating export file names...")
+        if os.path.isdir(EXPORT_DIR):
             for file in os.listdir(EXPORT_DIR):
                 if "@" not in file:
                     continue