Warning: setting attribute ‘:background’ of face ‘markdown-table-face’: nil value is invalid, use ‘unspecified’ instead.

The warning comes from [this line](https://github.com/nashamri/spacemacs-theme/blob/2ffca41e6e9aa55d8de5c40c6fb61e0d47763b5d/spacemacs-theme.el#L768), of course, but I don't understand how to change `nil` to `unspecified`. FWIW, I tried mimicking earlier lines but the only thing that worked for me was: `(markdown-table-face ((,class (:foreground ,base :background ,(unless head4-bg head4-bg 'unspecified)))))