Fix regression in AbstractJackson2Encoder

Closes gh-29274

spring-web/src/main/java/org/springframework/http/codec/json/AbstractJackson2Encoder.java

@@ -198,6 +198,7 @@ public abstract class AbstractJackson2Encoder extends Jackson2CodecSupport imple
 										bufferFactory.join(Arrays.asList(bufferFactory.wrap(prefix), dataBuffer)) :
 										dataBuffer);
 							})
+							.switchIfEmpty(Mono.fromCallable(() -> bufferFactory.wrap(helper.getPrefix())))
 							.concatWith(Mono.fromCallable(() -> bufferFactory.wrap(helper.getSuffix())));
 				}
 

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonEncoderTests.java

@@ -144,6 +144,14 @@ public class Jackson2JsonEncoderTests extends AbstractEncoderTests<Jackson2JsonE
 				.verifyComplete());
 	}
 
+	@Test
+	public void encodeNonStreamEmpty() {
+		testEncode(Flux.empty(), Pojo.class, step -> step
+				.consumeNextWith(expectString("["))
+				.consumeNextWith(expectString("]"))
+				.verifyComplete());
+	}
+
 	@Test // gh-29038
 	void encodeNonStreamWithErrorAsFirstSignal() {
 		String message = "I'm a teapot";