Fix exception during textInfos.moveToCodepointOffset when some characters correspond to 0 pythonic characters

[mltony]

Link to issue number:

Partially closes #16406.

Summary of the issue:

When trying to navigate by style an exception is thrown:

RuntimeError: Unable to find desired offset in TextInfo.

Description of user facing changes

N/A

Description of development approach

Investigation showed that in Outlook with word UIA enabled, sometimes there are characters that translate into 0 codepoint characters. To illustrate this further:

>>> p=focus.treeInterceptor.makeTextInfo('caret')
>>> p.expand('paragraph')
>>> p.text
'View in browser'
>>> p.collapse(end=True)
>>> p.move('character', -1, 'start')
-1
>>> p.text
''
>>> len(p.text)
0

E.g. we move a collapsed text info start by -1 character, and the resulting textInfo still appears to be empty- at least its Python text is empty. I anticipated this to happen somewhere, but didn't encounter a single example when I was implementing moveToCodepointOffset function, so I missed one edge case. Now fixing this edge case. Also adding unit tests.

Testing strategy:

Unit test. Tested on test case from the original issue with UIA enabled.

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.