Optimize the implementation of the svn_mergeinfo__to_string() API.
Optimize the implementation of the svn_mergeinfo__to_string() API.

* subversion/libsvn_subr/mergeinfo.c
  (svn_mergeinfo__to_string): If the input MERGEINFO hash is empty,
   return the empty string immediately.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion