lit-dom-expressions doesn't set value-less props as true

[afranchuk]

If you have

html`<${Foo} bar><//>`

the Foo component gets passed "" as the value of the bar property. To better match JSX behavior, I think it would be useful to have this value be true instead. It's especially error-prone since "" is falsy.

[trusktr]

Yeah perhaps true would make more sense here, to be consistent with <custom-element bar>