api: Make userTopics required in initial snapshot

Fixes-partly: #1837

Author: chrisbobbe

lib/api/model/events.dart

@@ -71,7 +71,6 @@ sealed class Event {
           case 'peer_remove': return SubscriptionPeerRemoveEvent.fromJson(json);
           default: return UnexpectedEvent.fromJson(json);
         }
-      // case 'muted_topics': … // TODO(#422) we ignore this feature on older servers
       case 'user_status': return UserStatusEvent.fromJson(json);
       case 'user_topic': return UserTopicEvent.fromJson(json);
       case 'muted_users': return MutedUsersEvent.fromJson(json);

lib/api/model/initial_snapshot.dart

@@ -35,8 +35,6 @@ class InitialSnapshot {
   @JsonKey(defaultValue: 10000)
   final int serverTypingStartedWaitPeriodMilliseconds;
 
-  // final List<…> mutedTopics; // TODO(#422) we ignore this feature on older servers
-
   final List<MutedUserItem> mutedUsers;
 
   // In the modern format because we pass `slim_presence`.
@@ -70,7 +68,7 @@ class InitialSnapshot {
 
   final UserSettings userSettings;
 
-  final List<UserTopicItem>? userTopics; // TODO(server-6)
+  final List<UserTopicItem> userTopics;
 
   final GroupSettingValue? realmCanDeleteAnyMessageGroup; // TODO(server-10)
 

lib/api/model/initial_snapshot.g.dart

@@ -307,7 +307,7 @@ class ChannelStoreImpl extends HasUserStore with ChannelStore {
     }
 
     final topicVisibility = <int, TopicKeyedMap<UserTopicVisibilityPolicy>>{};
-    for (final item in initialSnapshot.userTopics ?? const <UserTopicItem>[]) {
+    for (final item in initialSnapshot.userTopics) {
       if (_warnInvalidVisibilityPolicy(item.visibilityPolicy)) {
         // Not a value we expect. Keep it out of our data structures. // TODO(log)
         continue;

lib/model/channel.dart

@@ -1334,7 +1334,7 @@ InitialSnapshot initialSnapshot({
       emojiset: Emojiset.google,
       presenceEnabled: true,
     ),
-    userTopics: userTopics,
+    userTopics: userTopics ?? [],
     // no default; allow `null` to simulate servers without this
     realmCanDeleteAnyMessageGroup: realmCanDeleteAnyMessageGroup,
     realmCanDeleteOwnMessageGroup: realmCanDeleteOwnMessageGroup,