unreads: Take a missed opportunity for a debugLog to fire when helpful

For this debug check to work, it just needs a known read state for
one message.

When `oldUnreadsMissing` is false (the common case), the model holds
one of two answers for a message's read state: known-unread or
known-read.

When `oldUnreadsMissing` is true, the model holds known-unread or
unknown.

Before, we were skipping the debug check on seeing that
`oldUnreadsMissing` is true. That's earlier than we need to abort,
because the model might still hold a known-unread state for the
message. Now, we go ahead with the debug check if that's the case,
and still skip it otherwise.

Done by using our new `isUnread` helper, in this separate commit
because it's not NFC in debug mode.

lib/model/unreads.dart

@@ -265,9 +265,14 @@ class Unreads extends ChangeNotifier {
     //   https://chat.zulip.org/#narrow/stream/378-api-design/topic/mark-as-read.20events.20with.20message.20moves.3F/near/1639957
     final bool isRead = event.flags.contains(MessageFlag.read);
     assert(() {
-      final isUnreadLocally = _slowIsPresentInDms(messageId) || _slowIsPresentInStreams(messageId);
+      final isUnreadLocally = isUnread(messageId);
       final isUnreadInEvent = !isRead;
-      if (!oldUnreadsMissing && isUnreadLocally != isUnreadInEvent) {
+
+      // Unread state unknown because of [oldUnreadsMissing].
+      // We were going to check something but can't; shrug.
+      if (isUnreadLocally == null) return true;
+
+      if (isUnreadLocally != isUnreadInEvent) {
         // If this happens, then either:
         // - the server and client have been out of sync about the message's
         //   unread state since before this event, or