New Feature in Sass 3.5: content-exists()

[xzyfer]

A new function content-exists() will return true when called within a mixin that was passed content for use by the @content directive.

Spec https://github.com/sass/sass-spec/tree/content-exists/spec/sass_3_5/functions/content-exists
Sass issue sass/sass#1988
Sass PR sass/sass#2064