Let the user use ``RewriteCond %{...} !=""'' instead

of ``RewriteCond %{...} !^$'' which is ugly and has
bad performance.

Obtained from:
Submitted by:	Ralf S. Engelschall
Reviewed by:	Dean Gaudet, Ralf S. Engelschall


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@78852 13f79535-47bb-0310-9956-ffa450edef68