Fix NPE in ClientToProxyConnection while closing the client connection

src/main/java/org/littleshoot/proxy/impl/ClientToProxyConnection.java

@@ -866,7 +866,7 @@ private boolean shouldCloseClientConnection(HttpRequest req,
             }
         }
 
-        if (!HttpHeaders.isKeepAlive(req)) {
+        if (req != null && !HttpHeaders.isKeepAlive(req)) {
             LOG.debug("Closing client connection since request is not keep alive: {}", req);
             // Here we simply want to close the connection because the
             // client itself has requested it be closed in the request.