[PW-7980] Update NotificationEntity definitions and exception handling for notification processing

[candemiralp]

Summary

• NotificationEntity definitions updated to keep the consistency with the database.
• Exception handling logic updated. Nullable fields won't throw any exception while webhook processing.
• Type declarations added to modified classes.
• isReportNotification() check removed while receiving notification. All of the received notifications will be stored in the database to allow merchants for further custom implementation of the webhooks.

Tested scenarios

• REPORT_AVAILABLE notification w/wo merchant_reference
• AUTHORISATION notification
• REFUND notification

Fixes #345