[webcomponents] Clarify whether the empty string is a valid slot name or not (#319)

It's not clear from reading the spec what happens to an element with an empty slot name; e.g. `<span slot=""></span>` or a slot with an empty name e.g. `<slot name=""></slot`>`

Is the empty string considered as a valid slot name?  I think this at least deserves an informal note if not a normative clarification.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/319