The spec defines it as:

readonly attribute ViewTransitionNavigation navigation;

and Blink has:

readonly attribute CSSOMString navigation;

This is actually visible on the web, and Blink's behavior is tested inside a WPT:

https://searchfox.org/wubkat/rev/b36cbce69fddb7da33823f316bd8ead5bebee970/LayoutTests/imported/w3c/web-platform-tests/css/css-view-transitions/navigation/at-rule-cssom.html#24

The test expects this attribute to be an empty string, which is not possible if you have enumerated values (ViewTransitionNavigation), the CSSOM would return none with the enum instead of an empty string in this case.

cc @noamr @khushalsagar @mattwoodrow @vmpstr