fix(session): initialize $data property with default empty array

[woprrr]

Summary

• Initialize Session::$data with = [] to prevent TypeError when the property is accessed before initialization on a newly created session
• Replace isset($this->data) check in readData() with an explicit $loaded flag to preserve lazy loading from the session store
• Set $loaded = true in clear() and hydrate() to maintain consistent state

Fixes #273

Test plan

• All 28 existing SessionTest tests pass
• PHPStan level 6 with no errors
• PHP-CS-Fixer with no violations
• testSaveBeforeReadInitializesData confirms save() on a fresh session works
• testSessionLoadsDataFromStoreOnConstruction confirms lazy loading still works

[woprrr]

Rebased on top of current main — no conflicts.

The qa job failure is unrelated to this PR: php-cs-fixer reports 13 files, none of which are touched here (Session.php and SessionTest.php both pass locally). Looks like a CI-wide issue with the fixer rules version (3.95.1 in CI vs 3.94.2 in composer.lock).

[pgrimaud]

Thank for this PR @woprrr 😄
Could you fix the CS? 🙏

Thanks

[woprrr]

@pgrimaud it's impossible because it's not linked to my changes it from dev branch.