[cffi] Fix FFI.NULL to be CData (#15680)

Author: pacman82

stubs/cffi/cffi/api.pyi

@@ -19,7 +19,7 @@ class FFI:
 
     BVoidP: CType
     BCharA: CType
-    NULL: CType
+    NULL: CData
     errno: int
 
     def __init__(self, backend: types.ModuleType | None = None) -> None: ...