Correctly handle empty dangling volumes

When there's no dangling volumes 'None' is returned but code
is expecting List.  Convert to empty list.

docker_custodian/docker_gc.py

@@ -77,6 +77,8 @@ def get_all_images(client):
 def get_dangling_volumes(client):
     log.info("Getting dangling volumes")
     volumes = client.volumes({'dangling': True})['Volumes']
+    if volumes == None:
+        volumes = []
     log.info("Found %s dangling volumes", len(volumes))
     return volumes