@Test
public void conv_null_missing() {

  FunctionExpression conv = dsl.conv(DSL.ref(STRING_TYPE_MISSING_VALUE_FILED, STRING),
      DSL.ref(INT_TYPE_MISSING_VALUE_FIELD, INTEGER), DSL.literal(2));
  assertEquals(STRING, conv.type());
  assertTrue(conv.valueOf(valueEnv()).isMissing());
}