Ensure milestone titles are never empty

What does this MR do?

If a milestone title is empty, we cause a denial of service for the group milestone page because we raise an exception trying to generate a URL to a legacy milestone (which depends on title).

Easy fix: just validate the presence of title, which I'm surprised we weren't doing before this.

Screenshots

Currently 26 milestones exist with a blank title - should we deal with that?

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

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

Refs #12769 (closed)