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This is mostly aiming at allowing us to fiddle with the mock FS during
tests.

This is more convenient to set a FK to null when it's unset, rather than
having to write 2 different insert statements

This also ensures we are using the absolute path instead of the user
provided one

Ensure the files aren't cached anymore, and that the folder is indeed
deleted.

It only does binding a nullptr to sqlite, since we can't write to a
nullptr, nor can we assign nullptr to int/string/float/...