Fix: Change default value for 'description' in BigQuery_metadata_extractor results

[sahithi03]

Summary of Changes

Changed the default type of table and column description in BigQuery_metadata_extractor to None.
This change was made so that the table and columns descriptions entered through the UI persist in the backend db (mysql in our case).
Currently because the default is an empty string every time metadata is extracted from BigQuery the descriptions entered through the UI are overwritten by the empty string.
This can be avoided by making sure description is set to None if it does not exist.
https://github.com/amundsen-io/amundsen/blob/main/databuilder/databuilder/models/description_metadata.py#L75

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Documentation

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[sahithi03]

Hi @feng-tao, thanks for reviewing! Can I send a PR updating the patch version to have a new release of databuilder?

[feng-tao]

sure, I could help to trigger a release

[sahithi03]

Hi @feng-tao, I've created a PR #2056 for updating the patch version, please help trigger a release. Thanks!