fix(datastore): add constraint to prevent inserts of empty media types

What does this MR do?

This MR ensures that the media_type field in the media_types table cannot be an empty string. As we move toward dynamic media types, we need to be more cautious with invalid data, but we should probably avoid requiring strict adherence to RFC 6838 since media types are defined outside our control and registry makes only limited use of specific media types. This MR seeks to avoid primarily that uninitialized string values are not treated as a valid media type.

Related to Implement Dynamic Media Types with SMALLINT (#1258)

Author checklist

• Feature flags
  • Added feature flag:
  • This feature does not require a feature flag
• I added unit tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines
• For database changes including schema migrations:
  • Manually run up and down migrations in a postgres.ai production database clone and post a screenshot of the result here.
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    • I do not have access to postgres.ai and have made a comment on this MR asking for these to be run on my behalf.
  • Do not include code that depends on the schema migrations in the same commit. Split the MR into two or more.
• Ensured this change is safe to deploy to individual stages in the same environment (cny -> prod). State-related changes can be troublesome due to having parts of the fleet processing (possibly related) requests in different ways.

Reviewer checklist

• Ensure the commit and MR tittle are still accurate.
• If the change contains a breaking change, apply the breaking change label.
• If the change is considered high risk, apply the label high-risk-change
• Identify if the change can be rolled back safely. (note: all other reasons for not being able to rollback will be sufficiently captured by major version changes).

If the MR introduces database schema migrations:

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• If not, apply the label cannot-rollback.
• Add a section to the MR description that includes the following details:
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