Makes UmbNavItem.Published nullable

[AaronSadlerUK]

Updates the UmbNavItem.Published property to be nullable. This resolves an issue where custom item types were not working correctly.

• Fixes an issue preventing custom item types from functioning as expected in the navigation.
• Ensures that the Published property can handle null values, accommodating scenarios where publication status might be indeterminate.
• Includes new tests to validate deserialization of nullable UmbNavItem.Published property.

Relates to #105