Webform email handler default subject is missing title when email is sent.

Problem/Motivation

The mail.default_subject defaults to Webform submission from: [webform_submission:source-entity] and the webform_submission:source-entity] is blank when a form is submitted without a source entity.

Steps to reproduce (see attached webform)

• Create a new webform with a text field.
• Add an email handler with debugging enabled.
• Test the form
• Confirm email subject is "Webform submission from:"

Proposed resolution

Add and use new [webform_submission:source-title] token in the default email subject.

Remaining tasks

• Add token
• Test changes
• Provide update hook

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A