Tests: fixed reading QUIC streams on Perl < 5.24. The parse_stream() routine has had a missing explicit return if there were no streams received. In Perl < 5.24 this used to return no value, or an empty array in the list context. In modern Perl this returns an empty value, or an array of 1 element, which made the check for last index of the returned array work rather by accident. The fix is to return explicitly and to check the array size in callers instead.