We need treat a ``np.nan`` as a clip arg as ``None`` (which means don't clip on that side). This makes tihngs more user friendly with no loss of generality. ``` In [1]: Series([1,2,3]).clip(np.nan) ValueError: Cannot use an NA value as a clip threshold ``` this is easily fixed by inside ``.clip()`` by: ``` if isnull(lower): lower = None if isnull(upper): upper = None ```