Minor fixes and improvements in MUCTPI ByteStream conversion
A group of small fixes/improvements for the MUCTPI BS converter (four commits):
- Fix timeslice header encoding - add missing word identifier
- Encode nSlices as detector event type
- Fix handling of the offline Run-3 format flag in candidate word MSB
- Check for empty data and fill histograms before early exit
The first three affect only the xAOD->BS conversion so don't affect P1 data taking. The last one is relevant for P1, but it only changes the behaviour in case errors are found - no changes for good data.
Jira: ATR-25069