SPIGOT-7789: Fix NPE in CraftMetaFirework applyToItem

By: blablubbabc <lukas@wirsindwir.de>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaFirework.java

@@ -185,9 +185,11 @@ class CraftMetaFirework extends CraftMetaItem implements FireworkMeta {
         }
 
         List<FireworkExplosion> effects = new ArrayList<>();
+        if (hasEffects()) {
             for (FireworkEffect effect : this.effects) {
                 effects.add(getExplosion(effect));
             }
+        }
 
         itemTag.put(FIREWORKS, new Fireworks(power, effects));
     }