`extract_representative_frame` not work with TF without timeunit

[keighrim]

Bug Description

When a TimeFrame object doesn't have the following properties

• start/end (instead `targets)
• timeUnit (no time notation is being used anyway),
• representatives (for any reason) ,

trying to grab the representative timepoint from such a time frame falls back to grabbing the "middle" frame, then raise error because _get_mid_framenum implementation relies on timeUnit, start, and end props.

Reproduction steps

There's currently no pipeline that results in the described situation. This bug is found during debugging for clamsproject/app-doctr-wrapper#7.