Ensure `preferred_language` always have a value.

What does this MR do?

Users created before 9.2 can have the preferred_language empty (NULL). This way, on the User preferences screen, a user might update the their language to Bulgarian, the first language of the language selector. To avoid that, and possible other problems, the User#preferred_language is fall backing to I18n.default_locale, which is already the default value for new users.

• 6d5364cf User#preferred_language default value was introduced in 9.2
• Related to: #221037 (closed)
• Decision of this approach: !37064 (comment 382955398)
• Issue to add the constraint validation: #229504 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team