Add Empty inline

[cuihtlauac]

Fix #297

[shonfeder]

I'm slightly unsure what we gain by introducing the Empty data constructor here. If we want a unit value, that would already be available as Concat ([], []), and we could add, e.g., a named value as a shorthand for this: let empty attr = Concat (attr, [])

Meanwhile, adding a new data constructor will require additional cases during analysis that don't actually improve the logic afaict.

Do you perhaps have any examples in mind where an explicit Empty data constructor is helps clarify things that couldn't be addressed through my proposed empty value?

[cuihtlauac]

Sorry, I should have explained where this comes from.

The initial use case came from the need being able to have a link with an empty label (of type inline), filled later with CSS.

I fully agree using a shorthand, as you propose, is a better solution, as it avoids having two ways to represent the same thing (Empty and Concat ([], []). I had overlooked that.

I'll update the PR with your proposal.

[shonfeder]

Looks great. Thanks!