Reimbourses with store credits is not assigning credits

[kennyadsl]

Steps to reproduce

1. Create a shipped order associated to a registered user
2. Create a RMA with Reimbursement Type: Store Credits
3. Create a Return
4. Reimbourse the Order
5. Go to the admin user page under admin/users/:id/store_credits

Expected behavior

I should find the credits assigned to that user.

Actual behavior

No store credits are assigned to that user.

System configuration

Solidus Version: master (via sandbox)
Extensions in use: none

[kennyadsl]

This happens when there is no user in the database with the email spree@example.com. This has been already addressed into #2802 that will also close this issue when merged.

I'm just reporting the issue here for documentation and to help other users that find this strange issue.