Since UserDir is a raw argument, our config system allows it to be blank.

To save us all the hassle, just flat out reject this silliness.  We could
treat blank as ".", but let the user shoot themselves in the foot
explicitly (and don't give them any ideas on how to do it).

PR: 8472


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94790 13f79535-47bb-0310-9956-ffa450edef68