Fix PHP deprecation errors for PHP@8.1

File: custom-functions.php
- Change the null default value to 0 as the type is array|int.
File: rt-transcoder-handler.php
- Change the apikey to string before passing to the trim() function because it gives an error if a parameter is not of type string.
- Remove default arguments from the send_notification method because there should be no required arguments after default arguments in the method/function. Also added validation to convert it into an Array.
Also few other changes have been made to fix the phpcs errors.

rtCamp / transcoder