Reno produces anchor references for sections automatically now, and
those don't have URIs set up so we want to ignore them. If we do have
another target that does have a URI we want to include that, so test
whether we have the URI before including the target info in the
output.
Change-Id: I84f1b67b3078662f72c09c0f69bd09a45a7fe83e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>