`policy_exists` fails if no policies exist

[adamstauffer]

Trying to run policy_exists with ness6rest.Scanner when no policies exist returns the following.

In [14]: scan.policy_exists('test')
TypeError                                 Traceback (most recent call last)
<ipython-input-14-1cb8ff8a73e0> in <module>()
----> 1 scan.policy_exists('test')

/.../lib/python3.5/site-packages/nessrest/ness6rest.py in policy_exists(self, name)
    334         self.action(action="policies", method="get")
    335
--> 336         for policy in self.res["policies"]:
    337             if policy["name"] == name:
    338                 self.policy_id = policy["id"]

TypeError: 'NoneType' object is not iterable

could switch to using self.res.get("policies", [])