Runtime error caused by empty dataframe when --protocols option is used

[joeyshuttleworth]

... when you chose a particular protocol there is a pandas concatenate issue (see tb1.txt).
tb1.txt
Tested with:
python3 scripts/run_herg_qc.py ../25112022_MW1_FF/ --output_dir ../25112022_MW_FF_processed --protocols rvot --wells B20 2>./tb1.tx

[MichaelClerx]

Closed as part of #115