Similar to `test_grp` in `test_pwd`, add a test with null value in name

[Yaminyam]

Both getpwnam(name) and getgrnam(name) should return ValueErr if null is entered in the name value
In test_grp, the part is being tested, but in test_pwd, the part is not being tested.
Is it ok to write a PR that adds that test?

[mdboom]

Yes, I think a PR to add that test would be quite welcome. Thanks.