Typo in event receiver not null constraint

[caproven]

Found a typo in the event receiver constraint which inadvertently permits events with a null description. This isn't really an issue since:

• In GraphQL, EventReceiverInput has all fields required
• In REST API, the struct for receivers stores description as a string (default val ""). The value will therefore never be null/nil

Quick fix so worth addressing though.

https://github.com/sassoftware/event-provenance-registry/blob/998616ab24d6cc15cafdd56b1adbee39bd8da7e4/pkg/storage/schema.go#L33C73-L33C73