Provide meaningful warning for conref="" #3217
When I have
I've encountered this in a lot of different documents over the last few weeks. The current message is meaningless for an author who has not encountered this recently, and does not even list the source file, resulting in a lot of difficult debugging for most people.
This update checks for an empty value. If the attribute is empty, it generates a new (better) warning message, and ignores the reference so that we do not try to read the directory. (The conref processing already knows to skip an empty value.) With this update, the build now reports a more useful message: