fix: removed unnecessary optional return for CopyCutAction

lib/src/editor_toolbar_controller_shared/copy_cut_service/copy_cut_service.dart

@@ -9,5 +9,5 @@ abstract class CopyCutService {
   /// Get the CopyCutAction by the type
   /// of the embeddable (this type is decided by
   /// the property type of that class)
-  CopyCutAction? getCopyCutAction(String type);
+  CopyCutAction getCopyCutAction(String type);
 }

lib/src/editor_toolbar_controller_shared/copy_cut_service/default_copy_cut_service.dart

@@ -8,7 +8,7 @@ import 'copy_cut_service.dart';
 /// flutter quill plugins
 class DefaultCopyCutService extends CopyCutService {
   @override
-  CopyCutAction? getCopyCutAction(String type) {
+  CopyCutAction getCopyCutAction(String type) {
     return (data) => Embed.kObjectReplacementCharacter;
   }
 }