Update Request#parseNetworkResponse javadoc to disallow returning null

Request#parseNetworkResponse claims it's okay to return null if a response can't be parsed. In reality, it seems like this would almost guarantee an NPE either in CacheDispatcher or ExecutorDelivery, so unless someone has an entirely custom ResponseDelivery, they probably already are returning Response.error rather than null. We should verify this and consider updating the Javadoc accordingly.