monitorTarget.append(buf, "key3", () -> null);

        assertEquals("\"key1\":\"value1\",\"key2\":123,\"key3\":null", buf.toString());
    }

    // Test append with empty string
    public void test_append_emptyString() {
        StringBuilder buf = new StringBuilder();
        Supplier<Object> supplier = () -> "";

        monitorTarget.append(buf, "emptyKey", supplier);