ContentObject parameter is nullable
Contrary to documentation this parameter is null when the user does
not have access to the revision content. When null do nothing.
Also updated the documentation at
https://www.mediawiki.org/wiki/Manual:Hooks/ArticleAfterFetchContentObject
Bug: 68004
Change-Id: Ie0bd99ade9a336f7ed04734882f9830b6d8367d4