Fix NPE in DemonlordSlayer

[hannibal002]

totemPos can be null in DemonlordSlayer.entityJoinWorld.
This PR is just a workaround so that the error does not show up, not investigating how the NPE can occur.

java.lang.NullPointerException
	at net.minecraft.entity.Entity.func_174818_b(Entity.java:1126)
	at gg.skytils.skytilsmod.features.impl.slayer.impl.DemonlordSlayer$entityJoinWorld$1$1.invoke(DemonlordSlayer.kt:196)
	at gg.skytils.skytilsmod.features.impl.slayer.impl.DemonlordSlayer$entityJoinWorld$1$1.invoke(DemonlordSlayer.kt:187)
	at gg.skytils.skytilsmod.core.TickKt$tickTask$1$1.invokeSuspend(tick.kt:56)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at gg.skytils.ktx-coroutines.DispatchedTask.run(DispatchedTask.kt:106)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at net.minecraft.util.Util.func_181617_a(Util.java:19)