This revision is to make sure the first note created gets ID 0000, not 0001.

As it truns out, decrementing the ID in `$next_id_file` after the user has
closed the editor whenever the file is empty or contains only whitespace is
a really bad idea.

The same user might create another note in the meantime, which will incement
the ID in that file. And then, if that gets decremented again, the next note
this user creates will have the same ID as the previous one.

It's also really not important to ensure IDs are consecutive. Deleting and
merging create gaps already and that is fine. The `rebuild` command is
available to “fix” that on occasion.