Username guess in configure.php can result in "undefined array key" warning

[ChrisHardie]

When attempting to guess the username, if the remote origin URL is not defined, the configure.php script will throw an "Undefined array key" warning:

https://github.com/spatie/package-skeleton-php/blob/main/configure.php#L90

Since most users are expected to use the repo as a template this may be a wontfix, but thought I would note it.