Use fallback lexer when no language is specified

[helfper]

(Initially posted in gohugoio/hugo#8913)

Currently, when using code fences without language specified (and GuessLanguage is disabled), no CSS class or style is added to pre/code. I think it would be more useful to use the fallback lexer instead -- like it happens when GuessLanguage is used and the language isn't identified -- as it would use a consistent fore/background color without special CSS handling by the user.