calc fix: "array_key_exists...null given" warning

When there are some calc element in joined group that is not shown then php warning was produced in details and edit form (not in new):

Warning: array_key_exists() expects parameter 2 to be array, null given in /.../fabrik_element/calc/calc.php on line 136

Warning: array_key_exists() expects parameter 2 to be array, null given in.../fabrik_element/calc/calc.php on line 142