You can output a node for only the non-missing values by using the below tagset. This tagset is shipped as a default for SAS 9.1.

   proc template;
      define tagset tagsets.sasxnmis;
         parent = tagsets.sasxmog;
         notes "SAS-XML generic XML-Data - does not pad pcdata with blanks and
         produces no empty tag for MISSING";
      define event SASColumn;
      start:
         break          / if exists(MISSING);
         ndent;
         put     '<' ;
         put     NAME;
         break;
      finish:
         break          / if exists(MISSING);
         put '</'       ;
         put NAME       ;
         put '>'  CR    ;
         xdent;
         break;
      end;
      define event MLEVDAT;
         break          / if exists(MISSING);
         put ' value="' / if cmp(XMLDATAFORM, "ATTRIBUTE");
         put VALUE      / if cmp(XMLDATAFORM, "ATTRIBUTE");
         put '"'        / if cmp(XMLDATAFORM, "ATTRIBUTE");
         break          / if cmp(XMLDATAFORM, "ATTRIBUTE");
         put '>'        ;
         put VALUE      ;
         break;
      end;
      end; /* sasxmiss */
   run;

   /*run this once (it will write to sasuser) and then to specify it */
   /*on the libname                                                  */

   libname foo xml 'nomiss.xml' tagset=tagsets.sasxnmis ;
   data foo.test; x=1; y=.; z=3; run;

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