chore: Trim formatted username fallback

A user has a mxid with a trailing "-" which
becomes a whitespace here. We should
trim those whitespaces after formatting.

lib/src/user.dart

@@ -129,7 +129,7 @@ class User extends Event {
           words[i] = words[i][0].toUpperCase() + words[i].substring(1);
         }
       }
-      return words.join(' ');
+      return words.join(' ').trim();
     }
     return 'Unknown user';
   }