drivers/encx24j600: Allow empty iolist elements

This commits allows using netdev_driver_t::send() to be called with one or more
empty iolist elements. This fixes issue
https://github.com/RIOT-OS/RIOT/issues/11163

drivers/encx24j600/encx24j600.c

@@ -205,6 +205,10 @@ static void sram_op(encx24j600_t *dev, uint16_t cmd, uint16_t addr, char *ptr, i
     char* in = NULL;
     char* out = NULL;
 
+    if (!len) {
+        return;
+    }
+
     /* determine pointer addr
      *
      * all SRAM access commands have an