fix(GuildMember): mark joined as nullable and fix fetching for these cases

src/stores/GuildMemberStore.js

@@ -178,7 +178,7 @@ class GuildMemberStore extends DataStore {
 
   _fetchSingle({ user, cache }) {
     const existing = this.get(user);
-    if (existing) return Promise.resolve(existing);
+    if (existing && existing.joinedTimestamp) return Promise.resolve(existing);
     return this.client.api.guilds(this.guild.id).members(user).get()
       .then(data => this.add(data, cache));
   }