Failure to decode A-ASSOCIATE-AC when transfer syntax length is zero

[scaramallion]

Description

When an A-ASSOCIATE-AC response is received that includes a rejected context containing a transfer syntax item of zero length then decoding the PDU fails and the association is aborted.

Expected behaviour

The PDU should decode successfully.

Actual behaviour

Decode fails, association is aborted

[scaramallion]

Occurs because the empty value is converted to '' which fails

if uid is not None and not validate_uid(uid):
    raise ValueError()

Standard says rejected contexts shouldn't have their transfer syntax value tested (whoops).