ogc:PropertyIsNull

sql like filer

 ( ! (`t`.`the_geom` is null) or `t`.`ID` = '2' )

<ogc:Filter>
  <ogc:Or>
    <ogc:Not>
      <ogc:PropertyIsNull>
        <ogc:PropertyName>the_geom</ogc:PropertyName>
      </ogc:PropertyIsNull>
    </ogc:Not>
    <ogc:PropertyIsEqualTo>
      <ogc:PropertyName>ID</ogc:PropertyName>
      <ogc:Literal>2</ogc:Literal>
    </ogc:PropertyIsEqualTo>
  </ogc:Or>
</ogc:Filter>