users: Have userDisplayEmail handle unknown users

Like userDisplayName does.

And remove a null-check `store.getUser(userId)!` at one of the
callers... I think that's *probably* NFC, for the reason given in a
comment ("must exist because UserMentionAutocompleteResult"). But
it's possible this is actually a small bugfix involving a rare race
involving our batch-processing of autocomplete results.

Related: #716

Author: chrisbobbe

lib/model/store.dart

@@ -693,10 +693,13 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
     return byDate.difference(dateJoined).inDays >= realmWaitingPeriodThreshold;
   }
 
-  /// The given user's real email address, if known, for displaying in the UI.
+  /// The user's real email address, if known, for displaying in the UI.
   ///
-  /// Returns null if self-user isn't able to see [user]'s real email address.
-  String? userDisplayEmail(User user) {
+  /// Returns null if self-user isn't able to see the user's real email address,
+  /// or if the user isn't actually a user we know about.
+  String? userDisplayEmail(int userId) {
+    final user = getUser(userId);
+    if (user == null) return null;
     if (zulipFeatureLevel >= 163) { // TODO(server-7)
       // A non-null value means self-user has access to [user]'s real email,
       // while a null value means it doesn't have access to the email.

lib/widgets/autocomplete.dart

@@ -275,10 +275,9 @@ class _MentionAutocompleteItem extends StatelessWidget {
     String? sublabel;
     switch (option) {
       case UserMentionAutocompleteResult(:var userId):
-        final user = store.getUser(userId)!; // must exist because UserMentionAutocompleteResult
         avatar = Avatar(userId: userId, size: 36, borderRadius: 4);
-        label = user.fullName;
-        sublabel = store.userDisplayEmail(user);
+        label = store.userDisplayName(userId);
+        sublabel = store.userDisplayEmail(userId);
       case WildcardMentionAutocompleteResult(:var wildcardOption):
         avatar = SizedBox.square(dimension: 36,
           child: const Icon(ZulipIcons.three_person, size: 24));

lib/widgets/profile.dart

@@ -47,7 +47,7 @@ class ProfilePage extends StatelessWidget {
     final nameStyle = _TextStyles.primaryFieldText
       .merge(weightVariableTextStyle(context, wght: 700));
 
-    final displayEmail = store.userDisplayEmail(user);
+    final displayEmail = store.userDisplayEmail(userId);
     final items = [
       Center(
         child: Avatar(