A source code style tweak.

* subversion/mod_dav_svn/reports/update.c
  (validate_input_revision): Use NULL instead of SVN_NO_ERROR as the constant
    for a (dav_error *) no-error, like everywhere else.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543193 13f79535-47bb-0310-9956-ffa450edef68