Fix a crash caused by UUID properties

[ftchirou]

• Previously, a predicate such as \Note.id == UUID() (where id is of type UUID) would cause a crash. This is now fixed.
• This PR also adds a tiny improvement. The == operator would previously not work when comparing Identifiable objects if the left hand side was an optional. This is fixed as well. It's now possible to do this 👇

class Note: NSManagedObject {
  @NSManaged var attachment: Attachment?
}

class Attachment: NSManagedObject, Identifiable {
  @NSManaged var id: String
  ...
}

let attachment: Attachment = // ...
let predicate = \Note.attachment == attachment