[Bleeding] Added missing NPE protection to removeMetadata(). Fixes BUKKIT-933

By: rmichela <deltahat@gmail.com>

paper-api/src/main/java/org/bukkit/metadata/MetadataStoreBase.java

@@ -82,6 +82,7 @@ public abstract class MetadataStoreBase<T> {
     public synchronized void removeMetadata(T subject, String metadataKey, Plugin owningPlugin) {
         String key = cachedDisambiguate(subject, metadataKey);
         List<MetadataValue> metadataList = metadataMap.get(key);
+        if (metadataList == null) return;
         for (int i = 0; i < metadataList.size(); i++) {
             if (metadataList.get(i).getOwningPlugin().equals(owningPlugin)) {
                 metadataList.remove(i);