Fix NPE when loading players that don't have a skin

[leMaik]

This fixes an NPE when loading players without a skin:

Exception in thread "Scene Manager" java.lang.NullPointerException: Cannot invoke "se.llbit.util.mojangapi.PlayerSkin.getUrl()" because "skin" is null
	at se.llbit.chunky.renderer.scene.Scene.loadChunks(Scene.java:909)
	at se.llbit.chunky.renderer.scene.SynchronousSceneManager.loadChunks(SynchronousSceneManager.java:178)
	at se.llbit.chunky.renderer.scene.AsynchronousSceneManager.lambda$loadChunks$3(AsynchronousSceneManager.java:152)
	at se.llbit.chunky.renderer.scene.AsynchronousSceneManager.run(AsynchronousSceneManager.java:82)