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

See gh-47263

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

@@ -37,3 +37,11 @@ dependencies {
 	dockerTestImplementation("org.testcontainers:junit-jupiter")
 	dockerTestImplementation("org.testcontainers:testcontainers")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}
+
+tasks.named("compileDockerTestJava") {
+	options.nullability.checking = "tests"
+}