fix(gateway): skip empty dm partner registration casts (#2677)

fluxer_gateway/src/session/session_dm_partners.erl

@@ -29,12 +29,18 @@ register_all(State) ->
 register_guild(_GuildId, GuildPid, State) ->
     case enabled(State) of
         true ->
-            cast_partners({GuildPid, undefined}, partner_ids(State), State),
-            State;
+            maybe_cast_partners({GuildPid, undefined}, partner_ids(State), State);
         false ->
             State
     end.
 
+-spec maybe_cast_partners(term(), [user_id()], session_state()) -> session_state().
+maybe_cast_partners(_GuildRef, [], State) ->
+    State;
+maybe_cast_partners(GuildRef, Partners, State) ->
+    ok = cast_partners(GuildRef, Partners, State),
+    State.
+
 -spec handle_mutual(guild_id(), [user_id()], session_state()) -> {noreply, session_state()}.
 handle_mutual(GuildId, PartnerIds, State) ->
     case maps:get(GuildId, maps:get(guilds, State, #{}), undefined) of

fluxer_gateway/test/session_dm_partners_tests.erl

@@ -41,6 +41,13 @@ register_guild_sends_direct_partners_to_that_guild_test() ->
         ?assertEqual({update_dm_partners, ?SESSION_ID, [2, 3]}, receive_cast())
     end).
 
+register_guild_without_direct_partners_stays_silent_test() ->
+    with_flag(true, fun() ->
+        State = (state(#{}))#{channels => #{300 => channel(3, [?SELF, 5, 6])}},
+        _ = session_dm_partners:register_guild(?GUILD, self(), State),
+        ?assertEqual(none, receive_cast())
+    end).
+
 register_all_is_inert_when_disabled_test() ->
     with_flag(false, fun() ->
         _ = session_dm_partners:register_all(state(connected())),