Cache null/empty returns for findOneBy

[dciprian-petrisor]

On 'findOneBy' calls which return no an empty result set from the persister, null values are not cached.
This makes it so that repeat calls of 'findOneBy' with the same criteria, always hit the database.

The changes introduced in this commit make the AbstractEntityPersister cache the empty result set in the region, so that repeat calls do not hit the database again.

The existing mechanism to invalidate the query cache is sufficient; any persist/update performed to the region will invalidate it, exactly how it happens for non-empty result sets.

This is also more in-line with the way 'loadByCriteria' and 'loadAll' behave.

Fixes #11563

[dciprian-petrisor]

Ill be adding/fixing tests here in the next few days

[VincentLanglet]

Isn't this a BC break ?

Before, I could have

$result = $repository->findOne(['foo' => $foo]);
$result2 = $repository->findOne(['foo' => $foo]);

And getting result2 !== null because of concurrency and another process could have inserted the result in DB.

After this PR it will still return NULL. So I cannot check again my DB.

[petrisorciprian-vitals]

@VincentLanglet If a value is inserted in DB, the timestamp region is updated.
The findOneBy query uses the timestamp region, so the NULL cached value is no longer valid because the timestamp in the timestamp region is greater than the one in the cached NULL result.

It is no different than caching a value.