Micropub updates: accept empty arrays

[barryf]

https://sentry.io/barryf/transformative/issues/334158093/

Transformative::Micropub::InvalidRequestError: Invalid request: the 'add' property should be a hash.
  lib/transformative/micropub.rb:58:in `verify_hash'
    raise InvalidRequestError.new(
  lib/transformative/micropub.rb:36:in `action'
    verify_hash(properties, 'add')
  lib/transformative/server.rb:180:in `block in <class:Server>'
    post = Micropub.action(params)
...
(45 additional frame(s) were not displayed)

Transformative::Micropub::InvalidRequestError: Invalid request: the 'add' property should be a hash.