Fix: Stream classes' id field is always serialized

A stream class' "id" must not be expressed in the trace metadata
if the trace's packet header does not contain a stream_id field.
Otherwise, the reader will try to ensure the packets' stream_id
corresponds to the stream class' id and will fail to do so.

The fix is to omit the serialization of the stream class' id
field entirely if the stream_id field is not found within the
trace's packet header.

It remains unclear whether or not a stream class has an actual
"id" in this case, or if some "unspecified" id state should
be implemented.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>