Fix text decoration shorthand

Jonathan Neuhauserrequested to merge
joneuhauser/extensions:fix-text-decoration-shorthand into master

What does the merge request do?

Fix a crash when querying a style containing the text-decoration shorthand. This was not covered in any of our tests, but showed up in inkscape/extras/extension-xaml!10 (merged).

Checklist

  • Add unit tests (if applicable)
  • Changes to inkex/ are well documented
  • Clean merge request history

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