Covering zero case

[alvarolivie]

• Fix price check to cover zero as well

Pre-Review Checklist

• I've read the CONTRIBUTING.md guide.
• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents to show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes, either structured or in the comments.
• I've run go generate . to ensure that the Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.78%. Comparing base (0c1bc45) to head (c850d38).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #677   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files         323      323           
  Lines       16247    16247           
=======================================
  Hits        15074    15074           
  Misses        837      837           
  Partials      336      336           

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