fix(buckets): fixed line 396 where if a bucket was null then retrieving its ID would cause a NPE

Author: seailz

src/main/java/com/seailz/discordjar/DiscordJar.java

@@ -393,6 +393,7 @@ public List<Bucket> getBuckets() {
 
     public Bucket getBucket(String id) {
         for (Bucket bucket : buckets) {
+            if (bucket == null) continue;
             if (bucket.id().equals(id)) return bucket;
         }
         return null;