[fix]: hictk convert: do not hang when processing .cool files with pixels with 0 interactions

[robomics]

See #378

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.14%. Comparing base (c79831b) to head (29ef792).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...ictk/hic/impl/interaction_to_block_mapper_impl.hpp	33.33%	0 Missing and 2 partials ⚠️

❌ Your patch status has failed because the patch coverage (33.33%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #379       +/-   ##
===========================================
+ Coverage   28.64%   77.14%   +48.50%     
===========================================
  Files         102      177       +75     
  Lines       13348    22813     +9465     
  Branches      830     2476     +1646     
===========================================
+ Hits         3824    17600    +13776     
+ Misses       9064     4151     -4913     
- Partials      460     1062      +602     

Flag	Coverage Δ
[tests	fuzzer](https://app.codecov.io/gh/paulsengroup/hictk/pull/379/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	28.55% <0.00%> (-0.10%)
[tests	integration](https://app.codecov.io/gh/paulsengroup/hictk/pull/379/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	68.62% <33.33%> (?)
[tests	unittests](https://app.codecov.io/gh/paulsengroup/hictk/pull/379/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	76.43% <33.33%> (?)

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