LookupInResult.contentAs() throws PathNotFound for exists result

Suggested release note:

The preferred way to get the result of an `exists` sub-document lookup is to call `LookupInResult.exists(index)`. However, it's also possible to get the result using `LookupInResult.contentAs(index, Boolean.class)`. Previously, `contentAs(index, Boolean.class)` was throwing `PathNotFoundException` for negative `exists` results instead of returning false. Now it returns false as you might expect.