ra_serf: Fix a bug that HTTP global proxy settings are not applied when 
http-proxy-exceptions is not set.

* subversion/libsvn_ra_serf/serf.c
  (load_config): Use empty string as default value for http proxy 
   exceptions list instead of NULL.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436253 13f79535-47bb-0310-9956-ffa450edef68