Handling null fields in the filtertransformer and validator

[ml-evs]

Closes #792, as reported by @JPBergsma.

This PR:

• adds validation that no filters on a field returns entries with null values for that field. This can occur in MongoDB as null values match filters that test for inequality (i.e. null != 1 evaluates to true).
• add additional "$ne": null expressions to all "$ne" filters in the MongoTransformer.
• add additional "exists": True expression to all "!=" filters in the ElasticsearchTransformer.
• add some null fields to our server test data so that we can verify this for other backends
• updates CODEOWNERS

[codecov]

Codecov Report

Merging #796 (0671b69) into master (44593e1) will decrease coverage by 0.04%.
The diff coverage is 71.42%.

❗ Current head 0671b69 differs from pull request most recent head 7ef9117. Consider uploading reports for the commit 7ef9117 to get more accurate results

@@            Coverage Diff             @@
##           master     #796      +/-   ##
==========================================
- Coverage   92.72%   92.68%   -0.05%     
==========================================
  Files          68       68              
  Lines        3669     3675       +6     
==========================================
+ Hits         3402     3406       +4     
- Misses        267      269       +2     

Flag	Coverage Δ
project	92.68% <71.42%> (-0.05%)	⬇️
validator	92.68% <71.42%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
optimade/validator/validator.py	82.14% <50.00%> (-0.25%)	⬇️
optimade/filtertransformers/elasticsearch.py	83.78% <100.00%> (ø)
optimade/filtertransformers/mongo.py	97.59% <100.00%> (+0.02%)	⬆️

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[ml-evs]

I'll wait for @JPBergsma to have a look at this before merging!

[JPBergsma]

I think Mongo DB uses null for missing values.
In the file mongo.py line 91 it should be null instead of None.
The same goes for lines 95,143,162,461 in test_mongo.py
Other than that I did not spot anything wrong.

[CasperWA]

I think Mongo DB uses null for missing values.
In the file mongo.py line 91 it should be null instead of None.
The same goes for lines 95,143,162,461 in test_mongo.py
Other than that I did not spot anything wrong.

The Python special NoneType value None is parsed as null for the MongoDB backend when needed through pymongo. So I think this should be fine.

Essentially, MongoDB null is represented as None in Python.

Or did you mean that it should be the string "null"?

[JPBergsma]

@CasperWA
Yes you are right. Pymongo does the conversion from None to null for us.

[JPBergsma]

Appart from the minor point in tests/server/routers/test_structures.py , I did not see any mistakes in this code. Although I did not fully understand the code in validator.py