Mark `targetDomainObject` as `@Nullable` in `PermissionEvaluator`

[therepanic]

Currently, there are other places where PermissionEvaluator is inherited and methods are overridden. However, these places aren't yet covered by jspecify. I expect that in the future, when these places are covered by jspecify, we'll ensure that @Nullable is used (the build simply won't pass).

Closes: gh-18259