Validate mandatory query field structure_features

[CasperWA]

I would argue they all should still fail due to them not supporting the mandatory query on structure_features.

• COD: https://www.crystallography.net/cod/optimade/v1/structures?filter=NOT%20structure_features%20HAS%20ALL%20%22assemblies%22
• OQMD: http://oqmd.org/optimade/v1/structures?filter=NOT%20structure_features%20HAS%20ALL%20%22assemblies%22
• OMDB: http://optimade.openmaterialsdb.se/v1.0.0/structures?filter=NOT%20structure_features%20HAS%20ALL%20%22assemblies%22 Edit: fixed as reported in [OPTIMADE] Support for queries on structure_features httk/httk#32.

Originally posted by @CasperWA in #676 (comment)

The validator should fail an implementation if querying structure_features fails, since it is mandatory to support queries for this field (in the /structures endpoints).

[ml-evs]

So the reason we're not validating this at the moment is that the validator can never find any values to query for with structure features, as the list is empty for almost everyone. This might be an easy fix to replace an if test_value with an if test_value is None as the default value will always be [] for structure_features.

[CasperWA]

Why not just test for a value we know could be present (from the models)? The point is more that the implementation should at least return an empty array if there are no structures with the chosen value. I.e., they shouldn't fail with errors or similar.

[ml-evs]

Yup, that's what I'm trying to do now, just needs some extras added to the config.