Don't break DAV tests if the HTTPd version isn't set.

* subversion/test/cmdline/svntest/main.py
  (is_mod_dav_url_quoting_broken): Make the existence of the
   SVNTEST_HTTPD_VERSION environment variable optional.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639519 13f79535-47bb-0310-9956-ffa450edef68