Highlighting indexer: Warn about empty attributes

Summary:
This patch extends the highlighting indexer to search for
attribute="". Referencing an empty attribute is most likely
an error.

In fact, this check finds one issue, and one false positive.
Still, it's better to fix it properly to ensure conformance.

Test Plan: make test

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9494