scheduled_email: Only call .save() if necessary.

Calling `email.save()` is only needed if we altered `email.address`;
it is unnecessary if we called `email.users.add(...)` which will have
done its own INSERT.

zerver/tests/test_signup.py

@@ -774,7 +774,7 @@ class LoginTest(ZulipTestCase):
         with queries_captured() as queries, cache_tries_captured() as cache_tries:
             self.register(self.nonreg_email("test"), "test")
         # Ensure the number of queries we make is not O(streams)
-        self.assert_length(queries, 89)
+        self.assert_length(queries, 87)
 
         # We can probably avoid a couple cache hits here, but there doesn't
         # seem to be any O(N) behavior.  Some of the cache hits are related

zerver/tests/test_users.py

@@ -771,7 +771,7 @@ class QueryCountTest(ZulipTestCase):
                         acting_user=None,
                     )
 
-        self.assert_length(queries, 84)
+        self.assert_length(queries, 82)
         self.assert_length(cache_tries, 27)
 
         peer_add_events = [event for event in events if event["event"].get("op") == "peer_add"]