Return default group MR approval settings

What does this MR do?

Return a default, in-memory group merge request approval settings when there is no existing one associate with the group. This change allows our front-end app on the Group settings page (!52297 (merged)) render correctly.

In the upcoming iterations, we will integrate the 'default' values with settings from parent group and instance.

curl -i -H Authorization\:\ Bearer\ YDYtSYhMQrz3AzpmdDqw \
  -XGET http\://localhost\:3000/api/v4/groups/28/merge_request_approval_setting

# Before
null

# After 
{
  "allow_author_approval": false
}

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - Behind a feature flag
• [-] Documentation (if required) - Behind a feature flag
• 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