[css-content-3] Add 'empty' keyword

[fantasai]

Using content: '' to make a pseudo-element exist feels pretty hacky, it might be nice to provide an empty keyword that computes to the empty string.

[xavortm]

Looking at how in other languages, defining an empty string also requires you to write variable = '', it feels the same and natural to me to keep it as content: ''. Other than that - would there be another use case for the empty keyword?