settings: Correctly handle passing empty anonymous group.

If empty anonymous group is passed for a setting value in
an API request, the setting is set to "Nobody" group.

zerver/tests/test_subs.py

@@ -708,6 +708,28 @@ class TestCreateStreams(ZulipTestCase):
         # testing another setting value.
         stream.delete()
 
+        nobody_group = NamedUserGroup.objects.get(
+            name="role:nobody", is_system_group=True, realm=realm
+        )
+
+        subscriptions = [{"name": "new_stream", "description": "New stream"}]
+        extra_post_data[setting_name] = orjson.dumps(
+            {"direct_members": [], "direct_subgroups": []}
+        ).decode()
+        result = self.subscribe_via_post(
+            user,
+            subscriptions,
+            extra_post_data,
+            allow_fail=True,
+            subdomain="zulip",
+        )
+        self.assert_json_success(result)
+        stream = get_stream("new_stream", realm)
+        self.assertEqual(getattr(stream, setting_name).id, nobody_group.id)
+        # Delete the created stream, so we can create a new one for
+        # testing another setting value.
+        stream.delete()
+
         subscriptions = [{"name": "new_stream", "description": "New stream"}]
         owners_group = NamedUserGroup.objects.get(
             name="role:owners", is_system_group=True, realm=realm
@@ -728,9 +750,6 @@ class TestCreateStreams(ZulipTestCase):
         stream.delete()
 
         subscriptions = [{"name": "new_stream", "description": "New stream"}]
-        nobody_group = NamedUserGroup.objects.get(
-            name="role:nobody", is_system_group=True, realm=realm
-        )
         extra_post_data[setting_name] = orjson.dumps(nobody_group.id).decode()
         result = self.subscribe_via_post(
             user,