A stylistic follow-up to r1627333.

* subversion/libsvn_ra_serf/blame.c
  (create_file_revs_body): Use SVN_VA_NULL rather than plain NULL.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1627483 13f79535-47bb-0310-9956-ffa450edef68