Reads the content of a URL (presumably, for a resource on the classpath) and generates a digest of its content. This digest will be incorporated into the URL provided to the client, to verify that the client has been "granted" access to this resource. This is only used for resources where {@link #requiresDigest(String)} istrue.
As of Tapestry 5.4, simply returns null.
@param url
@return the digest for the resource