Type check first parameter for Json::stringToObject()

[nibra]

Pull Request for Issue joomla/joomla-cms#37783

Replaces PR #61

Summary of Changes

If the first parameter in a call to Json::stringToObject() is not a string, an E_USER_DEPRECATED error is triggered. and the value cast to string. The next major version should typehint the parameter and remove the check.

Testing Instructions

Call the method with null as the first parameter. A log entry should be written to the deprecation log, if enabled. The return value should be an empty stdClass object.

Documentation Changes Required

No. The change actually just enforces the documented behaviour.