fix potential error

Author: yoelcortes

biosteam/_unit.py

@@ -251,7 +251,7 @@ def __init_subclass__(cls,
                 bst.units.__dict__[name] = cls
             if name not in bst.__dict__:
                 bst.__dict__[name] = cls
-        if hasattr(cls, '_finalize_doc'):
+        if hasattr(cls, '_finalize_doc') and cls.__doc__:
             cls._finalize_doc()
 
     ### Abstract Attributes ###