Allow requests with extra_params and no parameters

Without this commit, the following request:

post '/orders' do
  example_request 'should take an optional parameter hash', order_type: 'big' do
     ...
  end
end

would **not receive** the POST body/param `order_type: 'big'` specified.

The reason is that the function `params` was returning nil if
example.metadata[:parameters] was undefined, and the only one to
define example.metadata[:parameters] was to have previously called
"parameter". In other words, requests were not allowed to have so-called
"extra parameters" unless they had at least one "parameter" specified.

This commit removes this restrictions, allowing requests to specify all
their parameters "inline".