Improve background color

[joshmedeski]

Closes #424

Adjusting the get_background_color_of_highlight_group function to support cleared highlight groups.

• Falling back to highlight_group_normal.foreground is both confusing and makes it impossible to overwrite the default background value to a null value. So I've removed it.
• Removed the fallback else if background color isn't defined. In my case, I don't want there to be a background color so I'm clearing the OctoEditable highlight but the previous logic would still add a color back.

Does this pull request fix one issue?

Resolves https://github.com/pwntester/octo.nvim/discussions/424

Special notes for reviews

Please let me know if the black background was intentional. And if so, what I would need to do to allow the get_background_color_of_highlight_group to return a cleared value so I can get a transparent UI.

[pwntester]

Thanks! it looks good to me. I guess I was using black as the fall safe option, but it makes sense to allow the color to be cleared

[joshmedeski]

No, thank YOU! I love using this plugin, such a time saver.

And thanks for the quick merge.