HTTPCLIENT-1470 CachingExec(ClientExecChain, HttpCache, CacheConfig, AsynchronousValidator) throws NPE if config is null

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/branches/4.3.x@1570965 13f79535-47bb-0310-9956-ffa450edef68