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
corrected following Rob's suggestion

but still had to cast also 
$data[$name] = (array) $data[$name];
Merge pull request #1191 from jaanusnurmoja/patch-42

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