PixelConditionsConfig: Support jobs with no input file

Make the new-JO PixelConditionsConfig.py behave like its old-JO counterpart in InDetRecCabling.py. Namely, instead of crashing in jobs with no input file (and thus no run number), configure the latest Pixel cabling mapping.

This reflects exactly the behaviour of lines 32-33 and 105 in:
https://gitlab.cern.ch/atlas/athena/blob/release/22.0.91/InnerDetector/InDetExample/InDetRecExample/share/InDetRecCabling.py

The issue fixed by this MR is blocking migration of some Trigger code calling RegionSelector configuration to new-JO, because regSelTool_Pixel_Cfg() pulls in PixelConditionsConfig and Trigger online configuration has no run number or input file (because the configuration is created for any future runs).