Option to treat 0 and null the same

[silverwind]

I think it does not make much sense to treat 0 and null different like the module currently does.

If one wants to treat them the same using 7-color colorRange, there is a issue in the legend rendering when where the two colors are listed twice and the legend being cut-off because the module can not correctly handle 7 colors in it:

I think the only clean way to solve is to have a boolean option that makes 0 and null equal color so colorRange can be defined using six colors.

See https://github.com/go-gitea/gitea/pull/22157 for more details and a workaround I've added to fix the legend rendering.

[razorness]

I will take a look into this. Thanks for opening the issue.