While the related parser function sets the accumulated related pages as
extension data on the parser output
(see RelatedArticles\Hooks::onFuncRelated). The ParserClearState
handler, however, sets the empty list as a property, which is then
stored in the DB.
Changes:
* Make RelatedArticles\Hooks::onParserClearState use
ParserOutput#setExtensionData, mirroring ::onFuncRelated
* Add a unit test for the the handler
Bug: T115698
Change-Id: I3deaf1e8ee78944250c3f26315ee2450b444a035
