fix(StateChanged): Make StateChange field argument nullable

[EduCavalheiro]

Description

This PR addresses a regression introduced in PR 286 where the $field argument in StateChanged was made mandatory.

This caused existing applications that rely on implicit field resolution (by omitting the argument) to fail with a Too few arguments error.

Related to:

• 288

Changes

• The $field argument is now made nullable and given a default value (null) to restore backward compatibility with existing usage:

public function __construct( ?State $initialState, ?State $finalState, Transition $transition, $model, ?string $field = null){}

[freekmurze]

Could you rebase with main in order to get the tests passing?