ENH: Styler display of NaN / null Values
The `Styler` allows for null cells to be highlighted using `.style.highlight_null()` but a `nan` text string will also be displayed.

If a column is coloured using `.background_gradient()` it must be a numeric type otherwise numerical comparisons fail. This means that using a construction of the form `..fillna('')` on the column will create an error as the column is cast to an `object` type.

`nan` values can be removed in the rendered table using a hack of the form `.render().replace('nan','')` but it would be cleaner to use something along the lines of `.style.replace_null(value)` that could be used to tidy up the display of `nan` cells.