code: public void putEx(PdfName key, PdfObject value) {
          if (value == null) {
              return;
          }
          put(key, value);
      }
docs: Associates the specified <CODE>PdfObject</CODE> as value to the
  specified <CODE>PdfName</CODE> as key in this map.
  
  If the <VAR>value</VAR> is a <CODE>PdfNull</CODE>, it is treated just as
  any other <CODE>PdfObject</CODE>. If the <VAR>value</VAR> is
  <CODE>null</CODE> however nothing is done.
  
  @param key a <CODE>PdfName</CODE>
  @param value the <CODE>PdfObject</CODE> to be associated to the
  <VAR>key</VAR>