1. For new tracks, the file scanner will set ```genre_id``` for ```track``` rows with no genre as the placeholder "Unknown Genre", so all new tracks should have a genre
2. For existing tracks, a forced re-scan is needed in order to add the genre (if it exists) and set the placeholder if it doesn't

So, yes it is an issue as the ```genre_id``` can be null, but after a full scan there should theoretically be no nulls