Avoid error on getting users of group permission access entity

[hissy]

If the group already deleted, an exception occurred when using a group entity of the deleted group.

Exception Occurred: /Users/hissy/Repositories/concrete5/concrete/src/Permission/Access/Entity/GroupEntity.php:21 Call to a member function getGroupMembers() on boolean (0)

Steps to reproduce:

1. Create a group
2. Create a Workflow then set a notification to the group
3. Apply the workflow from full sitemap page
4. Delete the group
5. Create a workflow request with approving a page