For grouped operations, handle empty inputs/returns

[cbyrohl]

Fixes a recent problem from #62

[codecov]

Codecov Report

Patch coverage: 9.82% and project coverage change: -0.58% ⚠️

Comparison is base (d6f923d) 55.22% compared to head (cb29161) 54.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   55.22%   54.64%   -0.58%     
==========================================
  Files          52       54       +2     
  Lines        4978     5071      +93     
  Branches      986     1003      +17     
==========================================
+ Hits         2749     2771      +22     
- Misses       2020     2091      +71     
  Partials      209      209              

Files Changed	Coverage Δ
src/scida/customs/arepo/dataset.py	30.03% <4.54%> (-1.48%)	⬇️
tests/customs/test_arepo.py	11.30% <14.63%> (-0.75%)	⬇️
src/scida/helpers_misc.py	54.54% <40.00%> (-1.20%)	⬇️

... and 2 files with indirect coverage changes

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