Fix GH-9720: Null pointer dereference while serializing the response

When traversing the result array, we need to cater to `param_name`
possibly being `NULL`. Prior to PHP 7.0.0, this w

Fix GH-9720: Null pointer dereference while serializing the response

When traversing the result array, we need to cater to `param_name`
possibly being `NULL`. Prior to PHP 7.0.0, this was implicitly done
because `param_name` was of type `char*`.

Closes GH-9739.

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