Return empty list when there are no audience values

rocketrip-al · rocketrip-al · commit e8550c1b3cc1 · 2016-03-07T12:19:20.000-05:00
Was returning `[None]` when parsing:

```
&lt;saml:AudienceRestriction&gt;
    &lt;saml:Audience/&gt;
&lt;/saml:AudienceRestriction&gt;
```
diff --git a/src/onelogin/saml2/response.py b/src/onelogin/saml2/response.py
@@ -243,12 +243,8 @@ def get_audiences(self):
         :returns: The valid audiences for the SAML Response
         :rtype: list
         """
-        audiences = []
-
         audience_nodes = self.__query_assertion('/saml:Conditions/saml:AudienceRestriction/saml:Audience')
-        for audience_node in audience_nodes:
-            audiences.append(audience_node.text)
-        return audiences
+        return [node.text for node in audience_nodes if node.text is not None]
 
     def get_issuers(self):
         """
