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