minor, provide nul idiff ctor and predicate

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>
hygiene, for ease of use assign nul difference upon __make_evrrul()

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>
Merge branch 'code/hygiene'

* code/hygiene:
  hygiene, for ease of use assign nul difference upon __make_evrrul()
  minor, provide nul idiff ctor and predicate