Forbid empty custom properties

[nex3]

LibSass currently ignores custom properties without values, such as --foo:; and --foo:#{""};. Since CSS considers these invalid, LibSass should error out for them rather than ignoring them.

See sass/sass-spec#1249 and sass/sass-spec#1259

[nschonni]

What should the behaviour be if the interpolation leads to a empty set? I saw the #{""} example, but i'm thinking about #{"$variable-evaluating-to-null"}.

[nex3]

I'm not sure what you mean by "an empty set", but any interpolation that would cause the value of the custom property to be empty should be an error.

[nschonni]

Sorry, I just meant the case when the selector evaluates to a null CSS selector like --foo:;. I didn't see the test cases covering that, and was being super literal 😦 .