Improve test coverage in zip.go

[BooleanCat]

Please provide a brief description of the change.

Improve test coverage in zip.go

Which issue does this change relate to?

N/A

Contribution checklist.

Replace the space in each box with "X" to check it off.

• [x] I have read and understood the CONTRIBUTING guidelines
• [x] My code is formatted (make check)
• [x] I have run tests (make test)
• [x] All commits in my PR conform to the commit hygiene section
• [x] I have added relevant tests
• [x] I have not added any dependencies

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.79% :tada:

Comparison is base (5eb752b) 97.60% compared to head (1e497c9) 98.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #50 +/- ## ========================================== + Coverage 97.60% 98.40% +0.79% ========================================== Files 19 19 Lines 376 376 ========================================== + Hits 367 370 +3 + Misses 6 4 -2 + Partials 3 2 -1 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/BooleanCat/go-functional/pull/50/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Godkin)

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