In the patch code, allow readline() callback implementations to return a
NULL line, rather than forcing them to return an empty stringbuf if no
line could be read.

* subversion/libsvn_client/patch.c
  (readline_prop, readline_file, readline_symlink): Return a NULL line
   if nothing was read, instead of allocating an empty stringbuf.
  (readline): If a NULL line was read, return the static string "".


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436335 13f79535-47bb-0310-9956-ffa450edef68