handle empty zone

[superstar54]

If a zone has no child, set it to finish immediately.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.72%. Comparing base (5937b88) to head (f28be4f).
Report is 54 commits behind head on main.

Files	Patch %	Lines
aiida_workgraph/engine/workgraph.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #272      +/-   ##
==========================================
+ Coverage   75.75%   79.72%   +3.97%     
==========================================
  Files          70       65       -5     
  Lines        4615     4908     +293     
==========================================
+ Hits         3496     3913     +417     
+ Misses       1119      995     -124     

Flag	Coverage Δ
python-3.11	79.64% <92.00%> (+3.97%)	⬆️
python-3.12	79.64% <92.00%> (?)
python-3.9	79.68% <92.00%> (+3.94%)	⬆️

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