[w3c/push-api] Make it clear that a push message does not require a payload (#289)

The standard should **explicitly** state that you can send a push signal without the notification payload (and without the related headers used for the payload encryption).

**This fact is only implicit in the current standard.** However it is widely used in production by many services that were born before the payloads.  

The section `11.2.2 Receiving a Push Message`, point `5`,  says `If the push message contains a payload`. However I can't find the place where the option "no payload" is described.

I think that you should state that more clearly in order to avoid future bugs.



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