Bugfix for #3173

[derekashauer]

Set membership id and level name to empty string when membership does not exist for the order

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• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Resolves PHP code notices .

How to test the changes in this Pull Request:

1. Go to Memberships > Orders
2. Click "email" for an order where the user currently does not have an active membership and send

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

Fix code notice when sending order email to user with no active membership level

[dparker1005]

With PMPro v3.4 breaking templates up into separate classes, this PR needs to be updated to update the variables in the new PMPro_Email_Template_Payment_Receipt class before it can be merged