autotagger: match file name if the file has no title metadata

[mklein-de]

This improves autotagging of items without metadata

• [x] ~Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)~
• [x] Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• [x] Tests. (Very much encouraged but not strictly required.)