Fix FileETag None directive.

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 "".

(Backport of patch applied to httpd-2.0 as original 1.3 code has the
same flaws.)

PR: 12202
Submitted by: Andrew Ho <andrew@tellme.com>

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@96610 13f79535-47bb-0310-9956-ffa450edef68