[JUNGFRAU][CORRECT] Fix/store roi only if defined

Description

A bug was introduced here https://git.xfel.eu/detectors/pycalibration/-/merge_requests/762/diffs that lead to creating CONTROL and INSTRUMENT sections for ROI even if none was defined.

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

Reviewers

@kluyvert