Update processActionArguments in maec40.py

processActionArguments now returns arguments_list regardless if it is populated or a 0-length list.

Received error attempting to generate a MAEC report for a specific sample:
"Failed to generate MAEC 4.0.1 report: object of type 'NoneType' has no len(). Traceback pointed to line 48 of action.py in CYBOX module, attempting to find len of self.action_arguments.  Found processActionArguments in modules/reporting/MAEC40.py returned either the arguments_list, or None.  arguments_list is already initialized to a 0-length list, so modified processActionArguments to just return arguments_list.  Fixed the issue.