Add validation for project_feature

What does this MR do?

Split out from !23766 (comment 283467480):

This change adds a presence validation to project -> project_feature. We have had rare cases where project_feature was missing. We don't know why yet, but adding the validation may help with finding out.

• Issue with background: #34367 (closed)
• Related MR adding a migration to clean up and add missing records: !23766 (merged)

The association is initialized with defaults, so this should never blow up under normal circumstances.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry -> Not adding one here, !23766 (merged) has it for the migration
• 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.