Assume persistence on values cached on the Sensei Home API

[fjorgemota]

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Right now, we're using the cache group sensei/temporary to cache values for Sensei Home.

Even though that cache group is marked as non-persistent here: https://github.com/Automattic/sensei/blob/c50c2339b57308fa6f6e69a0892eedc361bdd4f1/includes/class-sensei.php#L350 - apparently there are some object cache implementations that ignore completely that function.

So, we should add an expiration (probably somewhat short, like, 1 hour or less?) on the wp_cache_set methods, and avoid saving passing null values to wp_cache_set, to avoid possibly issues when caching that.