Relax prev checkpoint invariance check

[slundqui]

Only check if prev checkpoint exists if there are longs/shorts outstanding

[codecov]

Codecov Report

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

Project coverage is 81.52%. Comparing base (1643282) to head (76ca03d).

Files	Patch %	Lines
...c/agent0/hyperfuzz/system_fuzz/invariant_checks.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1649      +/-   ##
==========================================
- Coverage   81.54%   81.52%   -0.02%     
==========================================
  Files          98       98              
  Lines        7412     7410       -2     
==========================================
- Hits         6044     6041       -3     
- Misses       1368     1369       +1     

Flag	Coverage Δ
unittests	81.52% <0.00%> (-0.02%)	⬇️

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