Add a `.empty?` helper method

[GavinJoyce]

This would return true if all keys in the result (other than links and meta) are empty. It will simplify commands where we do this:

def execute
  result = Serializers::Account.resource(inputs)

  if result[:accounts].empty?
    status :not_found
  else
    result
  end
end

As this will be possible:

def execute
  result = Serializers::Account.resource(inputs)

  if result.empty?
    status :not_found
  else
    result
  end
end