webclient: Don't call the sink callback if no data is available

This happened frequently for me with Docker Desktop's
/var/run/docker.sock on macOS.
But I believe it can happen on other environments, even with TCP.

Alternatively, this case can be handled by the callback
implementations.  But it's simpler to handle the corner case here.

netutils/webclient/webclient.c

@@ -982,7 +982,11 @@ int webclient_perform(FAR struct webclient_context *ctx)
                    * received file.
                    */
 
-                  if (ctx->sink_callback)
+                  if (ws->offset == ws->datend)
+                    {
+                      /* We don't have data to give to the client yet. */
+                    }
+                  else if (ctx->sink_callback)
                     {
                       ret = ctx->sink_callback(&ws->buffer, ws->offset,
                                                ws->datend, &ws->buflen,