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()

test/sandbox_test.py

@@ -36,6 +36,14 @@ def test_sandbox(client, servicer):
     assert sb.poll() == 42
 
 
+@skip_non_linux
+def test_app_attached_from_env(set_env_client):
+    sb = Sandbox.create("echo", "hi", app=app)
+    sb.wait()
+
+    assert sb.stdout.read() == "hi\n"
+
+
 @skip_non_linux
 def test_sandbox_mount(client, servicer, tmpdir):
     tmpdir.join("a.py").write(b"foo")