Add nullability annotations to tests in smoke-test/spring-boot-smoke-test-bootstrap-registry

See gh-47263

smoke-test/spring-boot-smoke-test-bootstrap-registry/build.gradle

@@ -25,3 +25,7 @@ dependencies {
 
 	testImplementation(project(":starter:spring-boot-starter-test"))
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}