There is a rare edge case where the elements in the formula may be linked to the current page. This happens when MathWikibaseConnector->fetchPageUrl returns false, i.e., when the linked element does not have a correct site link.
The fix adds another condition to MathNativeMML.php->addLinksToMathML, where empty links are ignored.
Also added a test to test this case.
Related: T134311