Fix FileETag None directive.

- Fix segfault on strlen computation on the empty string in vlv case
- If the etag is "", don't set the ETag header to be "" - leave the
  header NULL instead.

Andrew's patch would change ap_meets_condition to accept "", but Justin
thinks it would be better just to sidestep it all together and not set
ETag when it would be "".

PR: 12207
Submitted by:	Andrew Ho <andrew@tellme.com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96609 13f79535-47bb-0310-9956-ffa450edef68