Form error class is not applied if an empty message is sent to form_set_error

If an empty message such as

form_set_error('my_element');

Is sent the error is recorded but no error class is added to element enough the value exists in the error array. This appears to be a bug given that the error is recorded but the value returned causes the if statement to fail. Attached patch modifies the behaviour in _form_set_class to check that we have a none null value returned.

First git patch so hopefully it works correctly.