Don't write any bitrate attribute if it isn't known. As long as one
doesn't want automatic bitrate switching, playback can work just
fine even if it isn't set.

If strict standard compliance is requested, this is still considered
an error, since the attribute is mandatory according to the spec.

Based on a patch by Rodger Combs.
Signed-off-by: Martin Storsjö <martin@martin.st>