Fix problem with no docstring being None

manim/utils/decorators.py

@@ -30,7 +30,7 @@ def some_private_method(self):
         @internal  # does not do anything, don't use
         def _my_second_private_method(self): ...
     """
-    doc = getattr(f, "__doc__", "")
+    doc: str = f.__doc__ if f.__doc__ is not None else ""
     newblockline = "\n    "
     directive = f".. warning::{newblockline}"
     directive += newblockline.join(