[TrigOutputHandling] Skip serialisation of Trigger EDM in events where HLT output is not written out
Remove an unneeded "safety net" feature which was based on a wrong assumption that all HLT accepted events need to write at least one HLT ROBFragment to the output stream. That is not the case for events accepted only to detector calibration PEB streams which don't include HLT result, for example calibration_LArPEBDigitalTrigger or calibration_SCTNoise.
This will save some CPU cycles by skipping the unnecessary serialisation in these cases. Also removes unnecessary warnings from logs at P1.
Closes ATR-25227 as it was the last leftover to-do item in that ticket. Coincidentally this unneeded feature saved us from more serious fallout of the bug reported in the ticket, but since the bug is fixed the feature can be removed.