Fix wrong transcription counts in check messages

[TimJentzsch]

Relevant issue: N/A

Description:

For some old users, the gamma in the transcription check messages was off (by a lot). This is probably because some old posts don't have completion times for the submissions.

The gamma_at_time method now also includes completed submissions without a time in the count.

Checklist:

• [x] Code Quality
• [ ] Tests (if applicable)
• [x] Inline Documentation