Fix #531: Allow empty graphs in check_well_formed

[ECZ-Quantum-Labs]

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Context (if applicable): Fixes issue #531 where empty graphs were incorrectly rejected by check_well_formed()

Description of the change: - Added early return for empty graphs in _check_flow_general_properties

• Modified partial order check to allow empty graphs
• Test case from issue [Bug]: _check_flow_general_properties rejects empty graphs #531 now passes

Related issue:

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.84%. Comparing base (af1c16d) to head (9de31ea).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/flow/core.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #537      +/-   ##
==========================================
- Coverage   88.85%   88.84%   -0.01%     
==========================================
  Files          49       49              
  Lines        7135     7139       +4     
==========================================
+ Hits         6340     6343       +3     
- Misses        795      796       +1     

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