Fix exception when triggering style navigation commands in an empty document

[mltony]

Link to issue number:

Closes #16407

Summary of the issue:

Exception during style navigation command in an empty document

Description of user facing changes

N/A

Description of development approach

Detecting this case and returning early.

Testing strategy:

Tested on use case from the issue.

Known issues with pull request:

N/A

Code Review Checklist:

• [x] Documentation:
  • Change log entry
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• [x] Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• [x] UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• [x] API is compatible with existing add-ons.
• [x] Security precautions taken.

[seanbudd]

The issue reference seems wrong here - is this meant to fix #16405

[mltony]

No issue reference is correct in the original description. In that issue there's a PDF document made of screenshots, so it indeed appears as an empty document in NVDA.