[Clean-up] Make sure all Converters return UnsetValue

[michael-hawker]

Describe the bug

As per guidance: https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.dependencyproperty.unsetvalue#remarks

This issue has been marked as "needs attention 👋" due to no activity for 15 days. Please triage the issue so the fix can be established.

This issue has been marked as "needs attention 👋" due to no activity for 15 days. Please triage the issue so the fix can be established.

This issue has been marked as "needs attention 👋" due to no activity for 15 days. Please triage the issue so the fix can be established.

This issue has been marked as "needs attention 👋" due to no activity for 15 days. Please triage the issue so the fix can be established.