@Test
    public void serializeFieldNullToNull() {
        converter.configure(Collections.singletonMap(JsonConverterConfig.REPLACE_NULL_WITH_DEFAULT_CONFIG, false), false);
        Schema schema = SchemaBuilder.string().optional().defaultValue("default").build();
        Schema structSchema = SchemaBuilder.struct().field("field1", schema).build();
        JsonNode converted = parse(converter.fromConnectData(TOPIC, structSchema, new Struct(structSchema)));
        JsonNode expected = parse("{\"schema\":{\"type\":\"struct\",\"fields\":[{\"field\":\"field1\",\"type\":\"string\",\"optional\":true,\"default\":\"default\"}],\"optional\":false},\"payload\":{\"field1\":null}}");
        assertEquals(expected, converted);
    }
When schemas are disabled, fields are mapped to Connect maps.
connect/json/src/test/java/org/apache/kafka/connect/json/JsonConverterTest.java