Add-on store: don't show help button if not available (#15259)
Fixes #15257
Summary of the issue:
For an installed add-on the add-on store displays the help button, even if no help is available.
Description of user facing changes
Hide the help button if no help is available
Description of development approach
Fix typing and explicitly check if a help doc is available when determining if to display the help button
Testing strategy:
Test with the add-ons and STR in #15257
Known issues with pull request:
If there's no help available for the selected NVDA language or english, the help button will not be shown.
NVDA will not fallback to available languages or than english.
