Add precondition to spawn location setter in spawn events

[masmc05]

No description provided.

[mbax]

Example of issue to fix with this.

java.lang.NullPointerException: Cannot invoke "org.bukkit.craftbukkit.CraftWorld.getHandle()" because the return value of "org.bukkit.Location.getWorld()" is null
	at net.minecraft.server.network.config.PrepareSpawnTask$Preparing.tick(PrepareSpawnTask.java:233) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.network.config.PrepareSpawnTask.tick(PrepareSpawnTask.java:123) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.network.ServerConfigurationPacketListenerImpl.tick(ServerConfigurationPacketListenerImpl.java:218) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.network.Connection.tick(Connection.java:573) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.network.ServerConnectionListener.tick(ServerConnectionListener.java:246) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.MinecraftServer.tickConnection(MinecraftServer.java:1909) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.dedicated.DedicatedServer.tickConnection(DedicatedServer.java:582) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:1872) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1655) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.dedicated.DedicatedServer.tickServer(DedicatedServer.java:449) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.MinecraftServer.processPacketsAndTick(MinecraftServer.java:1713) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1383) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:304) ~[paper-26.1.2.jar:26.1.2-49-7799bf2]
	at java.base/java.lang.Thread.run(Thread.java:1474) ~[?:?]