Fix never use default local repository root issue

`strings.Split(value, "\000")` returns array with empty string (`[""]`) when key not found in gitconfig.

`localRepositoryRoots` check gitconfig `ghq.root` then compare result array length with 0, but it's length never be 0 now.

```go
	_localRepositoryRoots, err = GitConfigAll("ghq.root")
	utils.PanicIf(err)

	if len(_localRepositoryRoots) == 0 {
		usr, err := user.Current()
		utils.PanicIf(err)

		_localRepositoryRoots = []string{path.Join(usr.HomeDir, ".ghq")}
	}
```

 (at https://github.com/motemen/ghq/blob/master/local_repository.go#L163)

I try `ghq get motemen/ghq` without `ghq.root` setting then clone repository to `~/github.com/motemen/ghq` because never use default setting `~/.ghq`.

Thank you.