Volley to create valid NetworkResponse object when cache is disabled

* Add null check to request.CacheEntry() when creating NetworkResponse.

Currently when cache is not used, rather than creating a valid
NetworkResponse object, BasicNetwork.performRequest() throws NPE when
processing a http 304 no modified response. The NetworkResponse object,
if returned with the Volley error, can be used by the volley caller to
determine the http status code.

Change-Id: I007468ed98fb7ed5719a94f871323466e1bd0f8a