[#2021] Allow 0 length OpaqueDataTuple to be pack()ed
A customer was attempting to modify an Option 124 packet with zero length Opaque Data. While unpack() currently supports decoding zero length OpaqueDataTuples from the wire format, pack() seemed to have an explicit check to prevent the encoding of zero length OpaqueDataTuples. This seems like an oversight considering that some clients seem to send zero length Opaque Data.
The check has been removed from pack(), and tests have been updated to reflect this requirement.