Disable writing "0" to music files for total tracks/discs when values aren't available

I have "autotag: no" and "write: yes" in the config.yaml so beets won't add musicbrainz tags to my music files but the lastgenre plugin can still add genre tags. This works except that no matter what, beets still writes "0" to music file metadata for the total number of discs and the total number of tracks. So if "1" was previously the metadata tag for a track number, the new track number becomes "1/0", or if a track's disc number was previously tagged as "2", it becomes "2/0".

I prefer not to fill in these values in my library (which is why no values for total discs/tracks are imported into beets), but beets seems to require them in the database and thus also when writing to music files.

### Proposed solution

Either disable adding "0" in the beets library by default when no total number of discs/tracks is found on import, or add a configuration variable to prevent writing "0" to music files for these values.