`VersionConstraint.version` is allowed to be `None`

[Hritik14]

Is is more of a design decision than an issue.

Let's consider that in a place far far away, someone writes a piece code to get versions from all the constraints:

>>> vr = VersionRange.from_string("vers:npm/1.1|2.2")
>>> range_versions = [c.version for c in vr.constraints]
>>> range_versions
[SemverVersion(string='1.1'), SemverVersion(string='2.2')]

Everything is nice and good until we meet vers:ANY/*

>>> vr = VersionRange.from_string("vers:npm/*")
>>> range_versions = [c.version for c in vr.constraints]
>>> range_versions
[None]

Suddenly, range_versions break all expectations and contains a value value of NoneType than Version
A possible fix would be to allow None versions, such as Version(None)

Introduced here:
https://github.com/nexB/univers/blob/83e4fa651da7868f5c6f7c05a006bfadaac8d1cd/src/univers/version_constraint.py#L70

Related:
#10
#19