repo: properly handle NoneType in Default/Remote equality checks

BUG=none
TEST=none

Change-Id: I4ccdbbc7ba4b6f6e20c6959db1b46fdb44ea2819
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/308982


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jack Neus <jackneus@google.com>