Properly handle empty usernames

lib/pages/instance.dart

@@ -351,7 +351,10 @@ class _AboutTab extends HookWidget {
               ),
             ),
             ...site.admins.map((e) => ListTile(
-                  title: Text(e.preferredUsername ?? '@${e.name}'),
+                  title: Text((e.preferredUsername == null ||
+                          e.preferredUsername.isEmpty)
+                      ? '@${e.name}'
+                      : e.preferredUsername),
                   subtitle: e.bio != null ? MarkdownText(e.bio) : null,
                   onTap: () => goToUser(e.id),
                   leading: e.avatar != null

lib/pages/users_list.dart

@@ -28,7 +28,10 @@ class UsersListPage extends StatelessWidget {
         ),
         body: ListView.builder(
           itemBuilder: (context, i) => ListTile(
-            title: Text(users[i].preferredUsername ?? '@${users[i].name}'),
+            title: Text((users[i].preferredUsername == null ||
+                    users[i].preferredUsername.isEmpty)
+                ? '@${users[i].name}'
+                : users[i].preferredUsername),
             subtitle: users[i].bio != null
                 ? Opacity(
                     opacity: 0.5,