Can I add a comment to this?
The reason they must be referred to as unisons is because the notes are on the same staff position, and they have the same note name. This distinguishes them from chromatic semitones, or seconds, etc.
I would say that in music theory the B is called the note (I know that 'note' may mean something different in lilypond), the sharps and flats are called accidentals, and in music theory the octave is generally not given.
There's a music theory group over at StackExchange:
[But you will likely find better experts here!]
This is not entirely academic. Consider the Archicembalo, of which a historical example exists, and there are modern copies. It is a harpsichord designed by Trasuntino for 31 ET, a valid temperament, and it actually had 36 notes per octave. You can hear one here:
So you can actually play these unisons on such an instrument.
Given that this instrument was built in 1555 (!) I would not return a void or null for these intervals in the code, but the actual name. Just my thoughts on this interesting OT topic.
Andrew