fix: restore support for Slice Zones without choices

[angeloashmore]

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR fixes a bug where Slice Zones with no choices resulted in invalid syntax.

Checklist:

• [ ] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [x] All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Merging #55 (3744c91) into main (e23c465) will not change coverage. The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files          20       20           
  Lines        1464     1464           
  Branches      160      161    +1     
=======================================
  Hits         1421     1421           
  Misses         42       42           
  Partials        1        1           

Impacted Files	Coverage Δ
src/lib/buildUnion.ts	100.00% <100.00%> (ø)