mod_dav_svn unconditionally sends </update-report>

Running 'svn diff' on a revision prior to the inclusion of the file
fails ungracefully.

$ svn diff -r 1234 doc/handbook/client.texi 
subversion/libsvn_ra_dav/util.c:332: (apr_err=175002, src_err=0)
svn: RA layer request failed
svn: REPORT request failed on /repos/svn/trunk/doc/handbook
subversion/libsvn_ra_dav/util.c:316: (apr_err=175002, src_err=0)
svn: The REPORT request returned invalid XML in the response: XML
parse error at line 1: xmlParseStartTag: invalid element name
.. (/repos/svn/trunk/doc/handbook)

Original issue reported by andk