SHOULD throw if subject is empty string (parsed from structured content mode)? #382

Describe the Bug
Spec states that the subject MUST be a non-empty string. In structured content mode HTTP.toEvent adds an empty subject string as undefined to the event and calling validate on the resulting event does therefore NOT throw.

In binary content mode, HTTP.toEvent adds the empty subject as subject: '' and a subsequent call on validate throws subject should NOT be shorter than 1 characters. This would be the correct behavior I assume.

Steps to Reproduce
Use the following snippet:

const event = HTTP.toEvent({
  headers: {
    'Content-Type': 'application/cloudevents+json'
  },
  body: {
    id: '0815',
    specversion: '1.0',
    type: 'my.event.type',
    source: 'my.event.source',
    subject: ''
  }
})

// event has subject: undefined

event.validate() // does not throw

Expected Behavior
event should have subject: '' and event.validate() should throw.

Additional context

property is the empty string here and therefore the parser is not executed

In general not sure if this should be an error on HTTP.toEvent already or only at validate