feat(api): Make TimeDimension fields nullable

[Emile-Filteau]

Check List

• Tests have been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Issue Reference this PR resolves

Fixes #6972

Description of Changes Made (if issue reference is not provided)

When querying a null TimeDimension through GraphQL, Cube returns a Cannot return null value for non-nullable field. When querying through the playground, fetching the value of a TimeDimension can return a null value if the underlying date column is null. The GraphQL tab of the playground also outputs a query that will crash if ran against the GraphQL API that looks like this :

query CubeQuery {
  cube {
    my_cube {
      id
      my_date {
        value // <---- will crash with the linked error
      }
    }
  }
}

I'm not sure if there are underlying implications of doing this, let me know if it does not makes sense.