drivers/enc28j60: Allow sending with empty chunks
drivers/enc28j60: Allow sending with empty chunks

This commit allows `netdev_driver_t::send()` of `enc28j60` to be passed an
`iolist_t` containing one or more empty elements.

mtausig/RIOT,toonst/RIOT,authmillenon/RIOT,toonst/RIOT,kYc0o/RIOT,x3ro/RIOT,rfuentess/RIOT,kYc0o/RIOT,josephnoir/RIOT,smlng/RIOT,aeneby/RIOT,aeneby/RIOT,josephnoir/RIOT,kYc0o/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,smlng/RIOT,mtausig/RIOT,miri64/RIOT,lazytech-org/RIOT,jasonatran/RIOT,yogo1212/RIOT,x3ro/RIOT,mtausig/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,smlng/RIOT,mfrey/RIOT,kaspar030/RIOT,josephnoir/RIOT,aeneby/RIOT,x3ro/RIOT,rfuentess/RIOT,josephnoir/RIOT,toonst/RIOT,ant9000/RIOT,cladmi/RIOT,cladmi/RIOT,kaspar030/RIOT,kYc0o/RIOT,rfuentess/RIOT,jasonatran/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,mfrey/RIOT,basilfx/RIOT,OTAkeys/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,OTAkeys/RIOT,authmillenon/RIOT,lazytech-org/RIOT,toonst/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,mfrey/RIOT,kYc0o/RIOT,cladmi/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,smlng/RIOT,jasonatran/RIOT,basilfx/RIOT,rfuentess/RIOT,aeneby/RIOT,cladmi/RIOT,lazytech-org/RIOT,yogo1212/RIOT,OlegHahm/RIOT,mtausig/RIOT,basilfx/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,mfrey/RIOT,OlegHahm/RIOT,ant9000/RIOT,mtausig/RIOT,aeneby/RIOT,yogo1212/RIOT,rfuentess/RIOT,authmillenon/RIOT,x3ro/RIOT,RIOT-OS/RIOT,mfrey/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,authmillenon/RIOT,lazytech-org/RIOT,miri64/RIOT,authmillenon/RIOT,josephnoir/RIOT,kaspar030/RIOT,smlng/RIOT,miri64/RIOT,basilfx/RIOT,cladmi/RIOT,OlegHahm/RIOT,x3ro/RIOT,toonst/RIOT,RIOT-OS/RIOT,basilfx/RIOT