[12.x] Support null parameter in BusFake::chain() method

[stevebauman]

In #56536, I forgot to update the default value of the $jobs parameter of the BusFake::chain() method to match the Dispatcher::chain() method. Sorry! 😢

framework/src/Illuminate/Bus/Dispatcher.php

Lines 161 to 167 in 568926f

	/**
	* Create a new chain of queueable jobs.
	*
	* @param \Illuminate\Support\Collection|array|null $jobs
	* @return \Illuminate\Foundation\Bus\PendingChain
	*/
	public function chain($jobs = null)

framework/src/Illuminate/Support/Testing/Fakes/BusFake.php

Lines 733 to 739 in 568926f

	/**
	* Create a new chain of queueable jobs.
	*
	* @param \Illuminate\Support\Collection|array $jobs
	* @return \Illuminate\Foundation\Bus\PendingChain
	*/
	public function chain($jobs)