Check if maintenance_mode setting is available

What does this MR do?

This MR introduces an extra check to handle the case when maintenance_mode has not yet been added to the database, but the application code has been updated, and it checks for this setting.

Follow-up of !54428 (merged).

Fixes #321734 (closed).

Screenshots (strongly suggested)

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