Fix polymorphed null error (#11571)

Content.Server/Polymorph/Components/PolymorphedEntityComponent.cs

@@ -10,7 +10,8 @@ namespace Content.Server.Polymorph.Components
         /// The polymorph prototype, used to track various information
         /// about the polymorph
         /// </summary>
-        public PolymorphPrototype Prototype = default!;
+        [DataField("prototype", required: true, customTypeSerializer:typeof(PrototypeIdSerializer<PolymorphPrototype>))]
+        public string Prototype = string.Empty;
 
         /// <summary>
         /// The original entity that the player will revert back into