Change GetWidgetsAsync to always return a valid list

[dkbennett]

Summary of the pull request

Changes GetWidgetsAsync to always return a list instead of a list or null. Returning null is what the underlying GetWidgets() returns and that seems to slip through the nullable type checking. This change ensures GetWidgetsAsync always returns a list of Widgets, or an empty list if there were errors or null was returned from GetWidgets().

This fixes the empty pin widgets dialog when there were no widgets because the list was null and not checked, causing the dialog to crash. Null check is no longer required and this method will always have a valid list.

References and relevant issues

Closes #2686

Detailed description of the pull request / Additional comments

• Updated GetWidgetsAsync to always return an empty list if GetWidgets returns null or there was an error.
• Updated callers of GetWidgetsAsync to no longer need to check for null where null was being checked.
• Updated GetPreviouslyPinnedWidgets to also return empty list instead of null.
• Updated callers of GetPreviouslyPinnedWidgets to no longer need to check for null.
• Updated comments on GetWidgetAsync to indicate it no longer can return null.
   

Validation steps performed

• Manually verified pinning works when there are no widgets.
• Pinned a widget, removed it, pinned another one.
• Verifed restarting devhome recovers the widgets as expected and they remain pinned.

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated