test_signup: Add temporary nocoverage marker.

This was introduced in 23530ac81e and
will be used soon, but isn't used yet.

zerver/tests/test_signup.py

@@ -3270,7 +3270,7 @@ class UserSignUpTest(InviteUserBase):
         common function for all signup tests to avoid code duplication; doing
         so will likely require adding new parameters."""
 
-        if not realm:
+        if realm is None:  # nocoverage
             realm = get_realm("zulip")
 
         client_kwargs: Dict[str, Any] = {}