Allow new cassettes to be recorded via fixture

This change allows the Betamax default workflow to be used via the
fixture. That workflow is:

- Write a test that uses Betamax but for which no cassette presently
  exists
- Run test which causes Betamax to create the cassette and record the
  interactions
- Re-run tests and ensure that no network activity has actually happened

Keystoneauth1's YamlJsonSerializer relied on yaml.safe_load to return
something other than None if a file was loaded. Unfortunately, if the
file is zero-length then it will return None which breaks the above
workflow.

Instead, let's check the return value and if it is not None, return it,
otherwise, simply return an empty dictionary as Betamax expects.

Change-Id: I5b7d01439f391e2ecb589850e90573c74cd38080
Closes-bug: #1670697

keystoneauth1/fixture/serializer.py

@@ -89,6 +89,10 @@ class YamlJsonSerializer(betamax.serializers.base.BaseSerializer):
 
     def deserialize(self, cassette_data):
         try:
-            return yaml.safe_load(cassette_data)
+            deserialized = yaml.safe_load(cassette_data)
         except yaml.error.YAMLError:
-            return {}
+            deserialized = None
+
+        if deserialized is not None:
+            return deserialized
+        return {}