[php-symfony] Allow non mandatory array value in Symfony

Created by: Cubid

Hello,

We have an issue when a parameter is not set in a get query. In file Service/JmsSerializer.php, the deserializeArrayString method does not check for null value.

If value is null, the returned data is an array with 1 element:

array(1) {
[0]=>
string(0) ""
}

This value the fails validation.

Returning null for a null input corrects the issue.

Mentionning contributors as requested: @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @ybelenko, @renepardon