Lazily get game thread when needed

[SirYwell]

Fixes #543

Spark reads null as the game thread at the point of initialization. I wasn't able to really figure out why that changed with 1.21.11, but given how the information is used, it makes sense to just use a supplier directly and access the game thread when needed.

Tested on Fabric for 1.21.11.

[TheDGOfficial]

I've had this happen in 1.21.10 as well today, it seems to be just way rarer in this version as I never had it before. Just wanted to add for reference, I'm aware that the commit is merged and will be included in next release most likely (although not for 1.21.10 i guess).

Stacktrace for reference:

[13:48:19] [spark-worker-pool-1-thread-1/ERROR]: Exception occurred whilst executing a spark command
java.lang.NullPointerException: Cannot invoke "java.lang.Thread.getId()" because "thread" is null
	at knot/me.lucko.spark.common.sampler.ThreadDumper$Specific.<init>(ThreadDumper.java:145) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.common.sampler.ThreadDumper$GameThread.setThread(ThreadDumper.java:132) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.common.sampler.ThreadDumper$GameThread.get(ThreadDumper.java:124) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.minecraft.plugin.MinecraftClientSparkPlugin.getDefaultThreadDumper(MinecraftClientSparkPlugin.java:77) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.common.command.modules.SamplerModule.profilerStart(SamplerModule.java:199) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.common.command.modules.SamplerModule.profiler(SamplerModule.java:146) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.common.SparkPlatform.executeCommand0(SparkPlatform.java:472) ~[spark-1.10.155-fabric.jar:?]
	at knot/me.lucko.spark.common.SparkPlatform.lambda$executeCommand$3(SparkPlatform.java:370) [spark-1.10.155-fabric.jar:?]
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:545) [?:?]
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:328) [?:?]
	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:309) [?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090) [?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614) [?:?]
	at java.base/java.lang.Thread.run(Thread.java:1474) [?:?]

On an unrelated note, Thread#getId is deprecated since Java 19. Completely irrelevant to this error and using non-deprecated threadId instead would not change anything most probably - but I assume it's deprecated for a good reason by the JDK devs.