On my Mac I am in group 5000 which apparently has no name
(I suspect because it is an LDAP group and I cannot reach the
LDAP server). Do not make the test fail in that case.

Fixes #14806

Change-Id: I56b11a8e86b048abfb00812eaad37802fd2adcc5
Reviewed-on: https://go-review.googlesource.com/20710Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>