dbms_xmlgen.getxmltype(context) returns null when the sql table is empty

I have this following function to convert table data to XML. However, whenever the table data is null the resulting XML is null. I want it to add the column names in case of empty table data. What changes should I make in this function.

function XmlToBlob(p_query varchar2) return blob is 
  ctx         dbms_xmlgen.ctxhandle;
  xml         xmltype;
  resultBlob  blob; 
begin
  ctx := dbms_xmlgen.newcontext(p_query);
  dbms_xmlgen.setNullHandling(ctx, dbms_xmlgen.empty_tag);
  xml := dbms_xmlgen.getxmltype(ctx);
  if (xml is not null) then
    resultBlob := xml.getBlobVal(NLS_CHARSET_ID('AL32UTF8'));
  else
    resultBlob := null;
  end if;
  return resultBlob;
end;