Tooltip默认过滤掉value为null 的内容，是否能够增加配置

[xile611]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

数值为null，仍然需要在tooltip中展示对应的文案，如下图的场景

What does the proposed API look like?

tooltip.showNil

相关代码

https://github.com/antvis/g2/blob/a79580977cd308c3f0b0e867607753750db5b87c/src/geom/mixin/tooltip.js#L426

https://github.com/antvis/g2/blob/a79580977cd308c3f0b0e867607753750db5b87c/src/geom/mixin/tooltip.js#L436

[hustcc]

@xile611 好的，我感觉是很不错的配置，建议你后面有这些 feature 诉求，直接提 PR 即可，这样的效率是最高的。