SamlauthUserSyncEvent::isAccountChanged() returns null by default

Problem/Motivation

The documented return type is bool, but it's actually NULL by default.

Steps to reproduce

In a test:

$this->assertFalse($event->isAccountChanged());

Proposed resolution

Cast the return type to bool/initialize it to FALSE.

Strict types would help, but kind of a BC break if you support subclassing those event classes.