@Test
    public void nullValueToJson() {
        // This characterizes the production of tombstone messages when Json schemas is not enabled
        Map<String, Boolean> props = Collections.singletonMap("schemas.enable", false);
        converter.configure(props, true);
        byte[] converted = converter.fromConnectData(TOPIC, null, null);
        assertNull(converted);
    }
When schemas are disabled, fields are mapped to Connect maps.
connect/json/src/test/java/org/apache/kafka/connect/json/JsonConverterTest.java