style: Empty placeholder type should be @final

mypy soon will have special narrowing for `@final` singleton types: python/mypy#15646

So, it is a good idea to use `@final` for such types.

Author: sobolevn

litestar/types/empty.py

@@ -2,12 +2,13 @@
 
 __all__ = ("Empty", "EmptyType")
 
-from typing import TYPE_CHECKING, Type
+from typing import TYPE_CHECKING, Type, final
 
 if TYPE_CHECKING:
     from typing_extensions import TypeAlias
 
 
+@final
 class Empty:
     """A sentinel class used as placeholder."""