[Python] Skip termination when engine is not initialized (#3040)

This PR skips the MLCEngine termination when the engine is
not initialized at all (e.g., failed during JIT compilation).
Previously there will be an extra error message saying
`'MLCEngine' object has no attribute '_ffi'` which is confusing
and misleading. Skipping the termination can eliminate this error
message.

Author: MasterJH5574

python/mlc_llm/serve/engine_base.py

@@ -659,6 +659,8 @@ def terminate(self):
         if hasattr(self, "_terminated") and self._terminated:
             return
         self._terminated = True
+        if not hasattr(self, "_ffi"):
+            return
         self._ffi["exit_background_loop"]()
         if hasattr(self, "_background_loop_thread"):
             self._background_loop_thread.join()