Follow-up to r1632530: Treat all C string source buffers of known length
equally in our string APIs.  If the length is given as 0, allow for the
source buffer to be NULL.

* subversion/include/svn_string.h
  (svn_string_create_empty,
   svn_stringbuf_create,
   svn_stringbuf_appendcstr,
   svn_stringbuf_remove): Update docstrings.

* subversion/libsvn_subr/string.c
  (svn_string_ncreate,
   svn_stringbuf_ncreate,
   svn_stringbuf_insert,
   svn_stringbuf_replace): Handle 0 length buffers explicitly and prevent
                           access to the source buffers in that case.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632638 13f79535-47bb-0310-9956-ffa450edef68