repo: improve duplicate default check

If one default is totally empty, we don't need to fail.

BUG=b:187795796
TEST=unit tests

Change-Id: Id226a7a7cd183dbdee58f4681b84885cc9211375
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/309102
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jack Neus <jackneus@google.com>