Add some null checks to UpdateGraphicsUniforms()

[Clayell]

Recently saw a user in the RP-1 server complaining about NRE spam when switching to a vessel in the map view or tracking station. Obviously, this doesn't fix the original issue, but this PR does fix that NRE spam.

Relevant NRE:

[EXC 20:47:23.111] NullReferenceException
	Scatterer.SkyNode.UpdateGraphicsUniforms () (at <54f2924209e243e1ae5e4af0d4a9ce81>:0)
	Scatterer.SkyNode.OnPreRender () (at <54f2924209e243e1ae5e4af0d4a9ce81>:0)
	UnityEngine.DebugLogHandler:LogException(Exception, Object)
	ModuleManager.UnityLogHandle.InterceptLogHandler:LogException(Exception, Object)
	UnityEngine.Debug:CallOverridenDebugHandler(Exception, Object)

This attached log is an abridged version of the log that the user sent, which contains the logs after the scene switch to the track station (the original log was too big to attach here)
abridged.log

Edit: The user reported that reinstalling scatterer fixed whatever issue they had.