Allow empty Host: header arguments.

Previously, request that sent:

GET / HTTP/1.1
Host:

would get a 400.  RFC 2616 specifically allows for a "blank" host field.

The read_request code properly handled this, but the fix_hostname in
vhost.c would cause the 400.  Now, simply return in fix_hostname when
we see a blank hostname rather than erroring out.

PR: 7441


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94655 13f79535-47bb-0310-9956-ffa450edef68