Allow non-existent pet-items nodes.

If someone spins up MobArena with a really old config-file or simply
removes the `pet-items` section, the plugin throws an NPE. This commit
fixes that by allowing the section to not exist.

It might be a good idea to log some helpful information, but let's wait
and see if this isn't good enough.

Fixes #606, closes #608.

Thanks Chew!

src/main/java/com/garbagemule/MobArena/ArenaMasterImpl.java

@@ -280,6 +280,9 @@ public class ArenaMasterImpl implements ArenaMaster
         spawnsPets.clear();
 
         ConfigurationSection items = settings.getConfigurationSection("pet-items");
+        if (items == null) {
+            return;
+        }
 
         for (String key : items.getKeys(false)) {
             EntityType entity;