AbstractJackson2Encoder encodes empty Flux to invalid JSON

[kzander91]

M6 introduced a bug in the AbstractJackson2Encoder (I think I have narrowed it down to #28401):
Returning an empty Flux from a WebFlux endpoint yields the following JSON:

]

I created a project that reproduces this here:
demo.zip
Unzip and run ./mvnw test, a test failure pops up:

Response body: ']'
[ERROR] zero  Time elapsed: 0.011 s  <<< ERROR!
org.springframework.core.codec.DecodingException: JSON decoding error: Unexpected close marker ']': expected '}' (for root starting at [Source: UNKNOWN; line: 1, column: 0])
        at com.example.demo.DemoApplicationTests.zero(DemoApplicationTests.java:19)

Open the pom and switch the Spring Framework version to 6.0.0-M5 and run again: Test is fixed, the logged response body is

[]

as expected.

The project has a controller:

@RestController
class TestController {

    @GetMapping("/zero")
    Flux<MyType> zero() {
        return Flux.empty();
    }

    @GetMapping("/one")
    Flux<MyType> one() {
        return Flux.just(new MyType("one"));
    }

    @GetMapping("/two")
    Flux<MyType> two() {
        return Flux.just(new MyType("one"), new MyType("two"));
    }

}

And a test that calls the controller methods:

@WebFluxTest
class DemoApplicationTests {

    @Autowired
    private WebTestClient webTestClient;

    @Test
    void zero() {
        webTestClient.get()
                .uri("/zero")
                .exchange()
                .expectBodyList(MyType.class).hasSize(0); // fails on M6 due to the invalid JSON response
    }

    @Test
    void zeroToString() {
        webTestClient.get()
                .uri("/zero")
                .exchange()
                .expectBody(String.class)
                .consumeWith(r -> System.out.println("Response body: '%s'".formatted(r.getResponseBody())));
    }

    @Test
    void one() {
        webTestClient.get()
                .uri("/one")
                .exchange()
                .expectBodyList(MyType.class).hasSize(1)
                .contains(new MyType("one"));
    }

    @Test
    void two() {
        webTestClient.get()
                .uri("/two")
                .exchange()
                .expectBodyList(MyType.class).hasSize(2)
                .contains(new MyType("one"), new MyType("two"));
    }

}