SELECT XMLQUERY('copy $doc := . modify
                for $i in $doc/xxx/*[not(.//text())]
                where (string-length($i) = 0 )
                return delete nodes $i
                return $doc' 
         passing xmltype('<xxx><rt></rt><a><hg>5</hg><b><c></c></b></a></xxx>') returning content).GETCLOBVAL() as df from  dual;

DF                                                                              
--------------------------------------------------------------------------------
<xxx><a><hg>5</hg><b><c/></b></a></xxx>

<xxx><a><hg>5</hg></a></xxx>