Skip empty systems

[svchb]

With open boundaries a system can be empty. Lets skip them in the IO step.

Extracted from #604

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.89%. Comparing base (a27700b) to head (eaa5d96).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/visualization/write2vtk.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   69.92%   69.89%   -0.03%     
==========================================
  Files          86       86              
  Lines        5120     5122       +2     
==========================================
  Hits         3580     3580              
- Misses       1540     1542       +2     

Flag	Coverage Δ
unit	69.89% <0.00%> (-0.03%)	⬇️

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[efaulhaber]

Doesn't look like a great solution to me, as VTK files are missing for some steps. But considering that VTK writing currently doesn't work for 0 particles, it's at least better than before.