Validator should not rely on SHOULD "meta" field "data_returned"

[ml-evs]

Unfortunately, in many places, the validator relies on meta->data_returned when checking the consistency of filters. We can get around this by just disabling these checks when data_returned is not provided by the implementation.

Reported by @rartino:

Isn't the data_returned optional on SHOULD level? It isn't super easy to produce it in my implementation at the moment. I will probably get it implemented rather soon, but databases working with huge datasets can have a very difficult time producing an exact count of the number of hits of a query. See, e.g., google's hit count.

Originally posted by @rartino in Materials-Consortia/providers#53 (comment)

[rartino]

A closely related issue is that even if the implementation returns meta->data_returned, one of the tests checks if an empty filter returns the same number for meta->data_returned and meta->data_available. This test appears to fail with a red checkmark if meta->data_available is not present. That field is on MAY level, so it is a quite major bug to fail implementations on missing that field.