code: public M removeNullValueAttrs() {
  		for (Iterator<Entry<String, Object>> it = attrs.entrySet().iterator(); it.hasNext();) {
  			Entry<String, Object> e = it.next();
  			if (e.getValue() == null) {
  				it.remove();
  				_getModifyFlag().remove(e.getKey());
  			}
  		}
  		return (M)this;
  	}
docs: Remove attributes if it is null.
  @return this model