BD-3837: Update behavior when external_id and alias aren't included

[lydia-xie]

Why are you making this change? (required)

Update behavior when external_id and alias aren't included in CSV import for email address identifiers

Related PRs, issues, or features (optional)

• BD-3837

Feature release date (optional)

• N/A

Contributor checklist

• I confirm that my PR meets the following:
  • I signed the Contribution License Agreement (CLA).
  • My style and voice follows the Braze Style Guide.
  • My content contains correct spelling and grammar.
  • All links are working correctly.
  • If I renamed or moved a file or directory, I set up URL redirects for each file.
  • If I updated or replaced an image, I did not remove the original image file from the repository. (For more information, see Updating an image).
  • If my PR is related to a paid SKU, third party, SMS, AI, or privacy, I have received written approval from Braze Legal.

Submitting for review

If your PR meets the above requirements, select Ready for review, then add a reviewer:

• Non-Braze contributors: Add braze-inc/docs-team as the reviewer.
• Braze employees: Have any relevant subject matter experts (like engineers or product managers) review your work first, then add the tech writer assigned to your specific vertical. If you're not sure which tech writer to add, you can add braze-inc/docs-team instead.

Thanks for contributing! We look forward to reading your work.

[rachel-feinberg]

Lgtm!