Fix `math domain error` when value is 0

[frankbryce]

math.log(0) is undefined, and I think the tool should allow values of 0 in bar charts. This is currently causing issues.

[jbordoe]

FYI, I tried running this, I get another error due to this line: https://github.com/mkaz/termgraph/blob/09e17cc3510b62969fc12075dd698817357a5a93/termgraph/module.py#L327

[tian2992]

I can confirm the color black error