Duplicated empty intervals

[AlexandreDecan]

Intervals can be provided to build new intervals. The constructor expects to receive a list of intervals or atomic intervals. If two empty atomic intervals are provided, the following assumption holds:

assert I.Interval(I.empty().to_atomic(), I.empty().to_atomic()) == I.empty()

but if two empty non-atomic intervals (i.e., non AtomicInterval instances) are provided, the equivalent assumption does not hold:

assert I.Interval(I.empty(), I.empty()) == I.empty()

The reason is that we check for emptiness only for AtomicInterval instances, and not for Interval ones.