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

See gh-47263

module/spring-boot-zipkin/build.gradle

@@ -44,3 +44,11 @@ dependencies {
 	testImplementation(testFixtures(project(":core:spring-boot-testcontainers")))
 	testImplementation("com.squareup.okhttp3:mockwebserver")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}
+
+tasks.named("compileDockerTestJava") {
+	options.nullability.checking = "tests"
+}