Return Bytes.EMPTY from DirectBufferedData.readBytes() if length is zero

[poulok]

Update the DirectBufferedData.readBytes(int length) method to return Bytes.EMPTY if the length of the returned Bytes objects is zero. Callers can safely check the length of the returned object today, but the PBJ library should return Bytes.EMPTY for empty instances as much as possible to enable callers to use bytes == Bytes.EMPTY as well.