Fix nuke disk getting lost when polymorphed holder is deleted (#36058)

* Delete original entity when polymorph is deleted

* Switch to EntityTerminatingEvent

* Add RevertOnDelete option to PolymorphPrototype

* Fix error on server shutdown while polymorphed

* Set RevertOnDelete to false by default

* AsNullable

* Revert "Set RevertOnDelete to false by default"

This reverts commit 087c43fbb923c9369c61c9d001e18814b3de3aca.

* Use pattern matching instead of .Value

Content.Server/Polymorph/Components/PolymorphedEntityComponent.cs

@@ -18,7 +18,7 @@ public sealed partial class PolymorphedEntityComponent : Component
     /// The original entity that the player will revert back into
     /// </summary>
     [DataField(required: true)]
-    public EntityUid Parent;
+    public EntityUid? Parent;
 
     /// <summary>
     /// The amount of time that has passed since the entity was created