[Jungfrau][Dark] Account for dark runs not taken by the MDL device

Description

This MR is just for my sanity, but it doesn't offer an operational use.

Medium and Low gain doesn't matter in burst mode runs. Burst mode is always in high gain and the first dynamic gain dark run is what matters the most.

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

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

Checklist:

Reviewers

@kluyvert