Add nullability annotations to tests in module/spring-boot-cache-test

See gh-47263

module/spring-boot-cache-test/build.gradle

@@ -33,3 +33,7 @@ dependencies {
 
 	testRuntimeOnly("ch.qos.logback:logback-classic")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}