Raise when a PointSet is initialized empty

[AhmetNSimsek]

Fixes #621

[codecov]

Codecov Report

Attention: Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 46.16%. Comparing base (e45a688) to head (60c20af).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
siibra/volumes/parcellationmap.py	0.00%	4 Missing ⚠️
siibra/locations/boundingbox.py	0.00%	2 Missing ⚠️
siibra/locations/pointset.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
+ Coverage   46.14%   46.16%   +0.01%     
==========================================
  Files          75       75              
  Lines        7171     7181      +10     
==========================================
+ Hits         3309     3315       +6     
- Misses       3862     3866       +4     

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