Fixing typo when prior dict is initialised

The purpose of this MR is to fix a typo in the prior dict where an empty dictionary should be returned if there are no calibration envelopes stored but by mistake am empty list is returned.

An additional test has been added to prevent this from happening again.

approved this merge request

enabled an automatic merge when the pipeline for 81464f1a succeeds

merged

mentioned in commit f4293201