Fix lots of warnings about `nil` being invalid as foreground/background value

[kenranunderscore]

I've seen lots of these warnings for various faces:

Warning: setting attribute ‘:background’ of face ‘whitespace-space’: nil value is invalid, use ‘unspecified’ instead. [3 times]

I fixed these by following the advice and using 'unspecified wherever I found nil for either :foreground or :background.

[roosta]

Brilliant! thanks for this :)