Fix for NoneType error when post_data had no params

When post_data had no params, har2grinder would fail with the following stack trace:

Traceback (most recent call last):
  File "har2grinder.py", line 202, in <module>
    main()
  File "har2grinder.py", line 138, in main
    page['grinder']['function_code'] += prepare_entry_request_call(entry)
  File "har2grinder.py", line 46, in prepare_entry_request_call
    for parameter in post_data.get('params'):
TypeError: 'NoneType' object is not iterable