`empty_trajs` classmethod

I didn't push this straight away because I wanted @hadim approval on the addition.

The rationale for this class method is to avoid raising errors when no data is found (I had one because I wanted to draw trajectories)...

There's an usage example there:
 `sktracker/detection/nuclei_detector.py`

So what d'you think?
It's easy enough to modify the various detection algorithms with the new method (instead of `pd.DataFrame([])`