fix: newsletter subscription

[trevorjob]

Motivation:

The newsletter_id column in the newsletter_subscriber table was previously set as NOT NULL, which imposed a constraint that every subscriber must be associated with a specific newsletter. However, we've identified scenarios where subscribers may not need to be tied to a specific newsletter upon initial subscription, allowing for greater flexibility in subscription management.

Changes Made:

• Updated the newsletter_id column in the newsletter_subscriber table to allow NULL values by modifying the column's constraint from NOT NULL to NULL.
• This change enables the creation of newsletter_subscriber records without requiring an immediate association with a specific newsletter.

  Testing Details:

• Verified that the database migration executes successfully, updating the column constraint as expected.
• Added new test cases to ensure that newsletter_subscriber records can be created with a NULL value for newsletter_id.
• Ran existing test cases to confirm that other functionalities related to the newsletter_subscriber table remain unaffected by this change.

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change) ​

Checklist:

• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [X] I have added tests to cover my changes.
• [X] All new and existing tests passed.