crayon/cli output does not render correctly when comment=""

[gmbecker]

I believe this is a still-alive case of #1843

When the comment chunk option is set to "", we (still) see the eroneous escaped ansi color codes in output.

To reproduce, set the comment option in vignettes/test/rendering.Rmd to "", and then build the articles for pkgdown itself. When you view the produced html page for that vignette, you will see:

Which you do not see with the current value of comment="#>"