Fix the isse of parsing empty as array of [null] in json request

This is not coherent to no json requests.

Now:

  { people: [{name: 'angelo', posts:[] }] }

In a normal http post is decoded as:

  { :people => [{:name => 'angelo', :posts => []} ]}

while in json request is decoded as:

  { :people => [{:name => 'angelo', :posts => nil }] }

With this PR in the case illustrated above, json request and html mail have the same decoded params
fixes #11023