🐛 Destination BQ Denormalized: handle null values in fields described by a $ref schema

[sherifnada]

What

#7736 added a partial fix for this case but didn't handle the situation where the field described by $ref is either null or not present. This PR handles this case and adds tests.

How

Only attempt to set the value of the property to a string if it is present and not null in the record being processed.

Recommended reading order

1. BigQueryDenormalizedRecordConsumer.java
2. BigQueryDenormalizedDestinationTest.java

Pre-merge checklist

Community member or Airbyter

• Grant edit access to maintainers (instructions)
• Secrets in the connector's spec are annotated with airbyte_secret
• Unit & integration tests added and passing. Community members, please provide proof of success locally e.g: screenshot or copy-paste unit, integration, and acceptance test output. To run acceptance tests for a Python connector, follow instructions in the README. For java connectors run ./gradlew :airbyte-integrations:connectors:<name>:integrationTest.
• Code reviews completed
• Documentation updated
  • Connector's README.md
  • Connector's bootstrap.md. See description and examples
  • Changelog updated in docs/integrations/<source or destination>/<name>.md including changelog. See changelog example
• PR name follows PR naming conventions

Airbyter

If this is a community PR, the Airbyte engineer reviewing this PR is responsible for the below items.

• Create a non-forked branch based on this PR and test the below items on it
• Build is successful
• Credentials added to Github CI. Instructions.
• /test connector=connectors/<name> command is passing.
• New Connector version released on Dockerhub by running the /publish command described here
• After the new connector version is published, connector version bumped in the seed directory as described here
• Seed specs have been re-generated by building the platform and committing the changes to the seed spec files, as described here

[sherifnada]

/publish connector=connectors/destination-bigquery-denormalized

🕑 connectors/destination-bigquery-denormalized https://github.com/airbytehq/airbyte/actions/runs/1442788641
✅ connectors/destination-bigquery-denormalized https://github.com/airbytehq/airbyte/actions/runs/1442788641