Fix NullMetadata performance issue

cc #44 #45

@sdepablos @luxemate Can you guys try that fix ?

I was able to confirm that the `NullMetadata::serialize` method was called even though those cache files were supposed to already exist. With that fix, `ClassMetadata::serialize` is not called anymore once the cache is created.

On my test endpoint, the response time (prod env, cache warmed) went down from 305ms to 275ms with that fix (simple `ab -n 100 -c 1`).