do_set_realm_property: Noop if value isn't really changing.

It makes no sense to do operations if the value isn't changing. In
particular, this creates RealmAuditLog entries and sends useless events.

zerver/actions/realm_settings.py

@@ -56,6 +56,9 @@ def do_set_realm_property(
     ), f"Cannot update {name}: {value} is not an instance of {property_type}"
 
     old_value = getattr(realm, name)
+    if old_value == value:
+        return
+
     setattr(realm, name, value)
     realm.save(update_fields=[name])
 

zerver/tests/test_events.py

@@ -3532,7 +3532,11 @@ class RealmPropertyActionTest(BaseAction):
         if vals is None:
             raise AssertionError(f"No test created for {name}")
         now = timezone_now()
+        original_val = getattr(self.user_profile.realm, name)
+
         do_set_realm_property(self.user_profile.realm, name, vals[0], acting_user=self.user_profile)
+
+        if vals[0] != original_val:
             self.assertEqual(
                 RealmAuditLog.objects.filter(
                     realm=self.user_profile.realm,