Doesn't work with modal forms not attached to a node

Problem/Motivation

If a webform is not attached to a node, no emails are sent.

Steps to reproduce

Create a Webform as a modal form.
Do not attach to a node.
Trigger the webform using a dialog link.

Proposed resolution

Patch WebformDigestQueue::processItem() and WebformDigestsMailHandler::sendMessage() to handle the case where `$webformSubmission->getSourceEntity()` returns null.