Fix WebTestClient list assertions

Prior to this commit, the `WebTestClient` could only accept non-nullable
types in its `expectBodyList<T>` Kotlin extension function.
This commit relaxes this requirements to allow for more assertions.

Fixes gh-36476

spring-test/src/main/java/org/springframework/test/web/reactive/server/WebTestClient.java

@@ -900,13 +900,13 @@ public interface WebTestClient {
 		 * List-specific assertions.
 		 * @param elementType the expected List element type
 		 */
-		<E> ListBodySpec<E> expectBodyList(Class<E> elementType);
+		<E extends @Nullable Object> ListBodySpec<E> expectBodyList(Class<E> elementType);
 
 		/**
 		 * Alternative to {@link #expectBodyList(Class)} that accepts information
 		 * about a target type with generics.
 		 */
-		<E> ListBodySpec<E> expectBodyList(ParameterizedTypeReference<E> elementType);
+		<E extends @Nullable Object> ListBodySpec<E> expectBodyList(ParameterizedTypeReference<E> elementType);
 
 		/**
 		 * Consume and decode the response body to {@code byte[]} and then apply

spring-test/src/main/kotlin/org/springframework/test/web/reactive/server/WebTestClientExtensions.kt

@@ -75,7 +75,7 @@ inline fun <reified B : Any> ResponseSpec.expectBody(): BodySpec<B, *> =
  * @author Sebastien Deleuze
  * @since 5.0
  */
-inline fun <reified E : Any> ResponseSpec.expectBodyList(): ListBodySpec<E> =
+inline fun <reified E : Any?> ResponseSpec.expectBodyList(): ListBodySpec<E> =
 		expectBodyList(object : ParameterizedTypeReference<E>() {})
 
 /**

spring-test/src/test/kotlin/org/springframework/test/web/reactive/server/WebTestClientExtensionsTests.kt

@@ -23,6 +23,7 @@ import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
 import org.reactivestreams.Publisher
 import org.springframework.core.ParameterizedTypeReference
+import org.springframework.http.MediaType
 import org.springframework.web.reactive.function.server.router
 import java.util.concurrent.CompletableFuture
 
@@ -95,6 +96,14 @@ class WebTestClientExtensionsTests {
 		verify { responseSpec.expectBodyList(object : ParameterizedTypeReference<Foo>() {}) }
 	}
 
+	@Test
+	fun `ResponseSpec#expectBodyList with null value`() {
+		WebTestClient
+			.bindToRouterFunction( router { GET("/") { ok().contentType(MediaType.APPLICATION_JSON).bodyValue("[null]") } } )
+			.build()
+			.get().uri("/").exchange().expectBodyList<Foo?>().hasSize(0)
+	}
+
 	@Test
 	fun `ResponseSpec#returnResult with reified type parameters`() {
 		responseSpec.returnResult<Foo>()