fix: slice check with null value

[eddycharly]

Explanation

Slice check with null value.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (48238dc) 18.94% compared to head (8f120e4) 18.92%.

Files	Patch %	Lines
pkg/engine/assert/parse.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
- Coverage   18.94%   18.92%   -0.02%     
==========================================
  Files          33       33              
  Lines        2497     2499       +2     
==========================================
  Hits          473      473              
- Misses       1999     2001       +2     
  Partials       25       25              

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