Followup to r1537555: Switch back to the SVN_VA_NULL macro for trunk.

* subversion/mod_dav_svn/mod_dav_svn.c
  (dav_svn__translate_name): Use SVN_VA_NULL as is the proper style for trunk.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537556 13f79535-47bb-0310-9956-ffa450edef68