ENH: `Styler.highlight_null` can accepts `subset` argument
As proposed in #31345 and as a follow-up PR for [#29118](https://github.com/pandas-dev/pandas/pull/29118#discussion_r340733887)

- [x] closes #31345
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Won't have time near-term to look.

On Tue, Mar 3, 2020 at 7:24 PM William Ayd <notifications@github.com> wrote:

> *@WillAyd* requested changes on this pull request.
>
> minor nit on annotation otherwise lgtm
>
> @TomAugspurger <https://github.com/TomAugspurger> care to look?
> ------------------------------
>
> In pandas/io/formats/style.py
> <https://github.com/pandas-dev/pandas/pull/31350#discussion_r387398995>:
>
> > @@ -1003,19 +1003,23 @@ def hide_columns(self, subset) -> "Styler":
>
>      def _highlight_null(v, null_color: str) -> str:
>
>          return f"background-color: {null_color}" if pd.isna(v) else ""
>
>
>
> -    def highlight_null(self, null_color: str = "red") -> "Styler":
>
> +    def highlight_null(self, null_color: str = "red", subset=None) -> "Styler":
>
>
> ⬇️ Suggested change
>
> -    def highlight_null(self, null_color: str = "red", subset=None) -> "Styler":
>
> +    def highlight_null(self, null_color: str = "red", subset: Optional[Union[Label, Sequence[Label]]]=None) -> "Styler":
>
>
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 Thanks, it's ok now. Thanks @immaxchen nice PR!