Account for __doc__ possibly being None

Signed-off-by: liamhuber <[email protected]>

pyiron_workflow/nodes/transform.py

@@ -379,7 +379,7 @@ def _build_inputs_preview(cls) -> dict[str, tuple[Any, Any]]:
 
     @classmethod
     def _extra_info(cls) -> str:
-        return cls.dataclass.__doc__
+        return cls.dataclass.__doc__ or ""
 
 
 @classfactory