Fix empty client behavior with app-attached sandboxes

Previously, if a clientless app was attached to a sandbox, it would
error with NoneType. This commit checks app._client to make sure it
exists before using it.

modal/sandbox.py

@@ -199,7 +199,7 @@ async def create(
             _experimental_gpus=_experimental_gpus,
         )
         if client is None:
-            if app:
+            if app and app._client:
                 client = app._client
             else:
                 client = await _Client.from_env()