Add nullability annotations to tests in module/spring-boot-webtestclient

See gh-47263

module/spring-boot-webtestclient/build.gradle

@@ -37,3 +37,7 @@ dependencies {
 	testRuntimeOnly("jakarta.servlet:jakarta.servlet-api")
 	testRuntimeOnly("org.springframework:spring-webmvc")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}