Fix undefined array-key warnings in HookableBusinessEvent and harden ScalarValueType error

Fix undefined array-key warnings in HookableBusinessEvent and harden ScalarValueType error: a task in bird-critic-1.5-full-300 (Harbor dataset). After a refactoring that removed empty() calls, HookableBusinessEvent::getWebhookPayload() could access array keys that were not set, producing PHP…

The task

After a refactoring that removed `empty()` calls, `HookableBusinessEvent::getWebhookPayload()` could access array keys that were not set, producing PHP warnings. The fix uses the null-coalescing operator (`??`) to default missing keys. Separately, `ScalarValueType` is updated to throw `FrameworkException` instead of…

Part of kochsnow/bird-critic-1.5-full-300.