IdPv3: WARN Attribute definition produced no attribute

[Rod Widdowson]

> Isn't it WARN a level to high for an expected event?

I would say so.  I have entered
https://issues.shibboleth.net/jira/browse/IDP-590 to track it.  

For (code) structural reasons which are not if interest, this might be
difficult to fix elegantly and we may have to always demote that to an INFO
- which still seems high for an expected situation.