Minor improvements to hictk rename-chromosomes

[robomics]

This edge case was detected while developing #217.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.83%. Comparing base (ff82e76) to head (4c017ed).
Report is 4 commits behind head on main.

Files	Patch %	Lines
...rc/hictk/rename_chromosomes/rename_chromosomes.cpp	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   75.85%   75.83%   -0.02%     
==========================================
  Files         140      140              
  Lines       11580    11584       +4     
  Branches     1311     1311              
==========================================
+ Hits         8784     8785       +1     
- Misses       1717     1720       +3     
  Partials     1079     1079              

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