Do not unpack if data is empty
Do not unpack if data is empty

Under normal operations I see a lot of log messages like this:
```*net.UDPConn[0xc000010098]: Error during Unpack: unexpected EOF```

It seems this is due to the Unpack function being called with and empty 'data'. This small changes causes the error condition to be ignored.
vapourismo/knx-go