@Test
    public void schemalessWithEmptyFieldValueToConnect() {
        // This characterizes the messages with empty data when Json schemas is disabled
        Map<String, Boolean> props = Collections.singletonMap("schemas.enable", false);
        converter.configure(props, true);
        String input = "{ \"a\": \"\", \"b\": null}";
        SchemaAndValue converted = converter.toConnectData(TOPIC, input.getBytes());
        Map<String, String> expected = new HashMap<>();
        expected.put("a", "");
        expected.put("b", null);
        assertEquals(new SchemaAndValue(null, expected), converted);
    }
When schemas are disabled, fields are mapped to Connect maps.
connect/json/src/test/java/org/apache/kafka/connect/json/JsonConverterTest.java