fix: handle empty strings, bull values, and incorrect dates for date_…

[njuguna-n]

…of_birth column

Description

Handle non-date values in the date_of_birth column

closes #162

Code review checklist

• Readable: Concise, well named, follows the style guide, documented if necessary.
• Documented: Configuration and user documentation on cht-docs
• Tested: Unit and/or e2e where appropriate
• Backwards compatible: Works with existing data and configuration or includes a migration. Any breaking changes documented in the release notes.

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.

[njuguna-n]

@witash I see we already have NULLIF on this column. I'll update the version of cht-pipeline used by the MoH deployment instead.