Fix rate limiter not shutting down if there is an empty bucket at shutdown (discord-jda#2080)

Author: Vankka

src/main/java/net/dv8tion/jda/internal/requests/ratelimit/BotRateLimiter.java

@@ -150,6 +150,9 @@ private void cleanup()
                 // If the requests of the bucket are drained and the reset is expired the bucket has no valuable information
                 else if (bucket.requests.isEmpty() && bucket.reset <= getNow())
                     entries.remove();
+                // Remove empty buckets when the rate limiter is stopped
+                else if (bucket.requests.isEmpty() && isStopped)
+                    entries.remove();
             }
             // Log how many buckets were removed
             size -= buckets.size();