add nullptr check for TrackParticle in MuonCreatorTool

Hi,

this MR adds a nullptr check for the TrackParticle inside the MuonCreatorTool. With this check, the rare MT crash in Trk::ParticleCaloExtensionTool::caloExtension is avoided and a warning is printed. It resolves ATLASRECTS-5574. Of course as a follow-up, it still needs to be understood why the track particle is null, but a seg fault is now avoided and the MT jobs run through without crash.

Adding @wleight to inform him, and tagging @wlampl since this should be included before turning on MuGirl in the MT validation campaign.

Best, Nico