Validator should not rely on meta->data_available

[ml-evs]

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.

Originally posted by @rartino in #675 (comment)