Improve the signature of json_decode function

[aturki]

The native json_decode function returns:

• null in case of an error
• an stdClass if the $assoc parameter is false
• an associative array if it's true

Since the safe implementation throws by default, the null value ia never returned. I thin we can improve the phpdoc return type of this function so that the return type is deduced depending on the value of $assoc parameter so that we can leverage better type checking using phpstan (https://phpstan.org/blog/phpstan-1-6-0-with-conditional-return-types)