Fix invalid entity reference errors in applyMobScaling

[KabanFriends]

📌 Description

This is a one-liner PR that fixes this error that appears semi-frequently on my server:
(This can happen with varied stack traces, sometimes it's Hytale's vanilla logic removing an entity, sometimes it's third party mods)

[World|default] Failed to run task!
java.lang.IllegalStateException: Invalid entity reference!
        at com.hypixel.hytale.component.Ref.validate(Ref.java:59)
        at com.hypixel.hytale.component.Store.__internal_getComponent(Store.java:1222)
        at com.hypixel.hytale.component.Store.getComponent(Store.java:1216)
        at ThirdPartyPlugin//com.azuredoom.levelingcore.utils.MobLevelingUtil.lambda$applyMobScaling$0(MobLevelingUtil.java:72)
        at com.hypixel.hytale.server.core.universe.world.World.consumeTaskQueue(World.java:904)
        at com.hypixel.hytale.server.core.universe.world.World.tick(World.java:403)
        at com.hypixel.hytale.server.core.util.thread.TickingThread.run(TickingThread.java:89)
        at java.base/java.lang.Thread.run(Thread.java:1474)
Caused by: java.lang.Throwable
        at com.hypixel.hytale.component.CommandBuffer.removeEntity(CommandBuffer.java:291)
        at com.hypixel.hytale.server.npc.systems.NPCPreTickSystem.steppedTick(NPCPreTickSystem.java:128)
        at com.hypixel.hytale.server.npc.systems.SteppableTickingSystem.tick(SteppableTickingSystem.java:53)
        at com.hypixel.hytale.component.system.tick.EntityTickingSystem.doTick(EntityTickingSystem.java:92)
        at com.hypixel.hytale.component.system.tick.EntityTickingSystem.tick(EntityTickingSystem.java:36)
        at com.hypixel.hytale.component.Store.tick(Store.java:1974)
        at com.hypixel.hytale.component.system.tick.ArchetypeTickingSystem.tick(ArchetypeTickingSystem.java:36)
        at com.hypixel.hytale.component.Store.tickInternal(Store.java:1930)
        at com.hypixel.hytale.component.Store.tick(Store.java:1900)
        at com.hypixel.hytale.server.core.universe.world.World.tick(World.java:406)
        ... 2 more

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🧪 Type of Change

Please check the types of changes your PR introduces:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Tests added / updated

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👩‍💻 Implementation Details

Before modifying the entity data in MobLevelingUtil#applyMobScaling, it makes sure that the entity reference is still valid.

🛠 Changes

• Added a validity check in MobLevelingUtil#applyMobScaling:

- if (npc.getReference() == null)
+ if (npc.getReference() == null || !npc.getReference().isValid())

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✅ Checklist

Go over all the following items and check them before submitting your PR:

• My code follows the project’s code style (simply run spotlessApply to apply formatting)
• I have performed a self-review of my own code
• I have commented my code where necessary
• My changes generate no new warnings

[AzureDoom]

LGTM