Fix for empty string class name

[tbje]

• classList.add("") throws an exception that is somehow swallowed.

[bwbecker]

I'd like to speak in favour of merging this commit ASAP. In my case, I got bit by creating a set of classes to add with code similar to the following:

      def myClasses(cond1: Boolean, cond2: Boolean) =
        cls := Seq(
            if (cond1) "cond1" else "",
            if (cond2) "cond2" else ""
          ).mkString(" ")

Of the four possible ways to call myClasses, one of them fails when the html is rendered:

        p(myClasses(false, true), "my paragraph").toString() ==> "<p class=\"cond2\">my paragraph</p>"

Or, in a simpler form (note the leading space in " cond2"):

        p(cls := " cond2", "my paragraph").toString() ==> "<p class=\"cond2\">my paragraph</p>"

Curiously, a trailing space in the class renders just fine.

[sake92]

@bwbecker IDK if this is more readable to you, but you can use Option[AttrValue] and Option[AttrPair]: https://scastie.scala-lang.org/9BDfZS9xQpq8xgrHcJityA (the last example)

Also, the 2.13's Option.when is really handy for these. 😃