Implement `InputNotes::default`

[Mirko-von-Leipzig]

InputNotes::new is fallible, but cannot fail when created with an empty set. Implementing Default should therefore be possible, and remove the need for

InputNotes::new(vec![]).expect("Empty notes cannot fail")`

style constructors. There are other similar constructs e.g. this one in notes to which this could also apply.

See here as an example.

[bobbinth]

Aren't these already implemented here and here?

[Mirko-von-Leipzig]

I'm blind; apologies.