Fix User::nick_in unconditionally returning if the cache is passed

To be precise, when the user passes the cache to the function, it won't attempt to retrieve the nickname via HTTP at all if the guild or member do not exist.

src/model/user.rs

@@ -789,9 +789,11 @@ impl User {
         #[cfg(feature = "cache")]
         {
             if let Some(cache) = cache_http.cache() {
-                return guild_id.to_guild_cached(cache).and_then(|guild| {
-                    guild.read().members.get(&self.id).and_then(|member| member.nick.clone())
-                });
+                if let Some(guild) = guild_id.to_guild_cached(cache) {
+                    if let Some(member) = guild.read().members.get(&self.id) {
+                        return member.nick.clone();
+                    }
+                }
             }
         }