Minor improvements to hictk zoomify

[robomics]

The division by zero was detected while developing #217.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 75.85%. Comparing base (ff82e76) to head (e4d9085).
Report is 8 commits behind head on main.

Files	Patch %	Lines
src/hictk/cli/cli_zoomify.cpp	20.00%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   75.85%   75.85%           
=======================================
  Files         140      140           
  Lines       11580    11580           
  Branches     1311     1311           
=======================================
  Hits         8784     8784           
  Misses       1717     1717           
  Partials     1079     1079           

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