Mass email says "user doesn't exist" when all users is selected

(Actually it's just with the current version on GitHub)

Steps to reproduce:
Send a mass email to "All users". The text doesn't matter.

It will say that the requested user doesn't exist.

This is because the function explode() in includes/acp_email.php returns an array with the input string, if the delimiter string can't be found, instead of an empty array.