QtHelp: Don't index HTML documents without the fullqthelp feature

Without the fullqthelp feature, the title and contents of HTML
documents can't be extracted, but the documents were still inserted
into the search index with an empty title and empty contents.

Skip them instead.

Task-number: QTBUG-150689
Pick-to: 6.12
Change-Id: I8fc55032107a6f09a260b5b40e64dd899217b755
Reviewed-by: Kai Köhne <kai.koehne@qt.io>