fix for PHP warning "is_file(): open_basedir restriction in effect"

Tracy constantly reported this error on my host:
````plain
PHP Warning: is_file(): open_basedir restriction in effect. File(/home/users/.../user/plugins/.gitkeep/.gitkeep.yaml) is not within the allowed path(s): (/home/users/.../:/usr/share/pear/) in .../web/system/src/Grav/Common/Config.php:237
````

it has probably something to do with executing is_file() for a file in a non-existing dir