SLOP-46: Fix null dereference and null string use in RetainedArticles

[tosfos]

Fixes fatal when WikiPageFactory returns null and PHP 8 TypeError when mBodytext is null.

Jira: SLOP-46

Changes:

1. Tools.php createRedirect: getWikiPageFactory()->newFromTitle() can return null (e.g. when a hook returns false). Check for null and throw MWException so the caller (Hooks::onPageDeleteComplete) can catch and log instead of fataling.
2. OverwrittenDeleteAction.php: $outputPage->mBodytext can be null. Use ?? '' when reading it for getOffsetOfSubmitButtonFieldLayout and showForm so preg_match_all and substr_replace do not receive null (PHP 8 TypeError/deprecation).