Allow empty lines with discounts or charges, fix empty check

[samlown]

Describe your changes

Fixes #424. Invoices can now be built without lines if discounts or charges are present.

Checklist before requesting a review

• 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 the 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 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 81.34%. Comparing base (930e513) to head (44f825c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
+ Coverage   81.18%   81.34%   +0.15%     
==========================================
  Files         231      231              
  Lines       11365    11369       +4     
==========================================
+ Hits         9227     9248      +21     
+ Misses       1777     1760      -17     
  Partials      361      361              

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