MuonPrdTest - Fix crashes in the digitization test when running with NSW

Hi,

this is a small patch to the central validation framework of the muon community. It's used to monitor & debug each step of the muon reconstruction. If digitization is run with the NSW layout, a dummy instance of the CSC_Decoder was created. Even though it was never initialized it generated a dummy dependency on CSC conditions data making the digi job to crash.

I've one question to @jchapman, @novak. Is it intended that no xAOD::EventInfo is available if the job is executed via Reco_tf.py --inputHisFile but it's created if Digi_tf.py is called?

Tagging for information: @mbarel, @mvozak, @minlin, @pscholer