🕷 Improve graphql_parser code and tests to avoid errors when metadata fields are missing

[MatMoore]

The code in graphql_parser.py is a bit flaky - recently we've seen several bugs caused by missing metadata fields breaking things.

Proposal

• Review the entity models, and make sure they are compatible with corresponding datahub models. Specifically, fields should only be required if they are required in the datahub model, as otherwise we get ValidationErrors when fetching entities from datahub
• Check for gaps in test coverage
• (If it is not too difficult) consider property testing using Hypothesis

[hjribeiro-moj]

Current coverage :