Consider an empty node_id list as an empty selection

[GianlucaFicarelli]

If node_id in the neuron_classes configuration is set to an empty list, it's now considered as an empty selection instead of selecting all the neurons.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.04%. Comparing base (240fe50) to head (672e6ca).

Files	Patch %	Lines
src/blueetl/repository.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   85.05%   85.04%   -0.01%     
==========================================
  Files          45       45              
  Lines        2509     2508       -1     
  Branches      495      495              
==========================================
- Hits         2134     2133       -1     
  Misses        309      309              
  Partials       66       66              

Flag	Coverage Δ
pytest	85.04% <86.66%> (-0.01%)	⬇️

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