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
Merge "ContentObject parameter is nullable"