Add nil check to Version.Equal

[radeksimko]

I avoided adding similar check to Compare as the meaning of any integer value there could be misleading, but I believe that the check in Equal is useful e.g. in test cases where nil can appear and equality is checked, either directly, or via library such as https://github.com/google/go-cmp which calls Equal on any struct via reflection.

Such comparisons panic without this patch and therefore don't provide useful output from tests.

[findkim]

This change improves the usability without any undesired/misleading results for the other operators, looks good!