Use SVN_VA_NULL instead of plain NULL in variable-arguments lists in all
remaining cases, as far as I can discover, following r1536307 (introducing
SVN_VA_NULL) and r1543394 (warning about missing or inappropriate sentinels for
some Subersion functions).

[Lots of implementation files]: Replace NULL with SVN_VA_NULL in function
calls with variable argument lists.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543477 13f79535-47bb-0310-9956-ffa450edef68