Fix issue with cursor being required in output

"SearchActorsOutput" contained a "cursor" property that wasn't
optional, even though the lexicon states it is.

Sources/ATProtoKit/Models/Lexicons/app.bsky/Actor/AppBskyActorSearchActors.swift

@@ -18,7 +18,7 @@ extension AppBskyLexicon.Actor {
     public struct SearchActorsOutput: Codable {
 
         /// The mark used to indicate the starting point for the next set of results. Optional.
-        public let cursor: String
+        public let cursor: String?
 
         /// An array of actors.
         public let actors: [ProfileViewDefinition]