Default array() type for target argument

This PR fixes the PHP warnings:

    PHP Warning:  array_keys() expects parameter 1 to be array, null given in /.../Idephix/src/Idephix/Idephix.php on line 113
    PHP Warning:  implode(): Invalid arguments passed in /.../Idephix/src/Idephix/Idephix.php on line 113

thrown if idxfile doesn't contain any environment.