Refine revoked getter and improve enumeration

Changed the `revoked` getter to a nilable type for consistent handling of null values in JSON serialization, avoiding potential bugs. Improved the `each` method in the in-memory JTI provider by utilizing block notation, simplifying code readability and maintainability. These enhancements streamline data handling and iteration in the codebase.

src/authly/access_token.cr

@@ -9,7 +9,7 @@ module Authly
     getter access_token : String
     getter token_type : String = "Bearer"
     getter expires_in : Int64 = ACCESS_TTL.from_now.to_unix
-    getter revoked : Bool = false
+    getter? revoked : Bool = false
     @[JSON::Field(emit_null: false)]
     getter refresh_token : String
     @[JSON::Field(emit_null: false)]

src/authly/in_memory_jti_provider.cr

@@ -9,7 +9,7 @@ module Authly
     end
 
     # Implement the each method to make the class enumerable
-    def each
+    def each(&)
       @revoked_tokens.each { |jti| yield jti }
     end