Add missing IAE check for PersistentDataContainer#has

paper-server/src/main/java/org/bukkit/craftbukkit/persistence/CraftPersistentDataContainer.java

@@ -57,6 +57,7 @@ public class CraftPersistentDataContainer implements PersistentDataContainer {
 
     @Override
     public boolean has(NamespacedKey key) {
+        Preconditions.checkArgument(key != null, "The provided key for the custom value was null"); // Paper
         return this.customDataTags.get(key.toString()) != null;
     }