@Test
    public void imported_field_references_are_treated_as_valid_field_with_missing_value() throws ParseException {
        var documents = createDocs();
        assertEquals(Result.TRUE, evaluate("test.my_imported_field == null", documents.get(0)));
        assertEquals(Result.FALSE, evaluate("test.my_imported_field != null", documents.get(0)));
        assertEquals(Result.FALSE, evaluate("test.my_imported_field", documents.get(0)));
        // Only (in)equality operators are well defined for null values; everything else becomes Invalid.
        assertEquals(Result.INVALID, evaluate("test.my_imported_field > 0", documents.get(0)));
    }
@author Simon Thoresen Hult
@author bratseth
document/src/test/java/com/yahoo/document/select/DocumentSelectorTestCase.java