Define DefaultMfeConfig as non nullable

[gciotola]

What I did

This pull request includes a change to the getMfeConfig.ts file to improve type safety and clarity in the DefaultMfeConfig type definition.

Improvements to type safety and clarity:

• packages/organization-config/src/getMfeConfig.ts: Modified the DefaultMfeConfig type definition to ensure non-nullability by wrapping the ValidConfigForOrganizationsInCommerceLayer['mfe'] type in a NonNullable utility type.

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes.
• You are NOT deprecating/removing a feature.