fix(grpc): return {} for google.protobuf.Empty responses

[kkopanidis]

Summary

• Return callback(null, {}) instead of null/undefined for all google.protobuf.Empty gRPC handlers.
• Fixes database startup crash on RegisterAdminRoute after the protobufjs 8 upgrade (Empty: object expected).
• Adds the missing success callback in router patchRouteMiddlewares.
• Aligns handler typings with Empty responses (Record<string, never> / Empty).

Test plan

• pnpm exec turbo run build --filter=@conduitplatform/core --filter=@conduitplatform/database --filter=@conduitplatform/router --filter=@conduitplatform/chat --filter=@conduitplatform/authorization
• Confirm database module stays up on standalone/docker startup (no RegisterAdminRoute crash)