feat: nil version equality

[KnutZuidema]

This changes the Equal method to return true for 2 nil values and false if any one of the values is nil. Previously passing a nil value would have resulted in a panic.

[adamperlin]

Just adding another comment in favor of this PR! I just ran into a panic when calling equal on a nil version. It would be great if this case was handled!

[mattfarina]

This flow makes sense. It's a minor optimization for some cases and it stops a panic from happening in some other places.

Note:

a := MustParse("1.2.3")
b := MustParse("1.2.3")

// a != b

To get equality with == you need...

a := MustParse("1.2.3")
b := a

// a == b

[MikeZappa87]

@mattfarina is it possible to cut a release with this?