Fix PHP deprecation errors for PHP@8.1

[Utsav-Ladani]

Fix PHP deprecation errors for PHP@8.1

Fixes done in this PR

• 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.

Related Issues

• PHP warnings with PHP8.1  #275

[vishalkakadiya]

LGTM!