Allow passing NULL for the config dir param in svn_config_walk_auth_data, 
which means using the default configuration directory path.

* subversion/libsvn_subr/config_auth.c
  (svn_config_walk_auth_data): Remove check for NULL config dir string.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507382 13f79535-47bb-0310-9956-ffa450edef68