Fix accidental type conversion in sluggable behavior.

This generated code:

    $slugNum = substr($object->getAccountId(), strlen($slug) + 1);
    if (0 == $slugNum[0]) {
        $slugNum[0] = 1;
    }

    return $slug2 . ($slugNum + 1);

will turn `$slugNum` into an array if substr returns false. By checking
with `===` against `'0'`, this is avoided.
Merge pull request #707 from gunnarlium/bugfix-sluggable-fails-when-string-is-empty

Fix accidental type conversion in sluggable behavior.