Fix colored hexdumps for zero-length field values

[MaxGroot]

Fix a (very, very) minor bug in dumpstruct for structure instances where field values are of length 0. In such situations, the coloring scheme of the hexdump currently 'hangs'. See the screenshot below:

main on the left, the branch for this PR on the right. The difference is in line 10.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 89.32%. Comparing base (6a6cde0) to head (581aff0).

Files	Patch %	Lines
dissect/cstruct/utils.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   89.50%   89.32%   -0.19%     
==========================================
  Files          21       21              
  Lines        1924     1929       +5     
==========================================
+ Hits         1722     1723       +1     
- Misses        202      206       +4     

Flag	Coverage Δ
unittests	89.32% <20.00%> (-0.19%)	⬇️

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