Replace instances of svn_stringbuf_ncreate() that create an empty stringbuf.

* subversion/libsvn_subr/subst.c
  (keyword_printf): Use svn_stringbuf_create_empty where appropriate.

* subversion/mod_dav_svn/deadprops.c
  (db_open): Same.

* subversion/tests/libsvn_subr/skel-test.c
  (get_empty_string): Same. The original code is even illegal.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632637 13f79535-47bb-0310-9956-ffa450edef68