A better solution than my previous commit as pointed out by Andre Malo.
mod_expires: Initialize ExpiresDefault to NULL instead of "" to
avoid reporting an Internal Server error if it is used without
having been set in the httpd.conf file. PR: 23748, 24459
Submitted: Andre Malo, Liam Quinn  <liam htmlhelp.com>
Reviewed: Andre Malo, Paul J. Reder


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101846 13f79535-47bb-0310-9956-ffa450edef68