Add nullability annotations to tests in smoke-test/spring-boot-smoke-test-data-rest

See gh-47263

smoke-test/spring-boot-smoke-test-data-rest/build.gradle

@@ -34,3 +34,7 @@ dependencies {
 
 	testRuntimeOnly("com.jayway.jsonpath:json-path")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}