Logic to verify the terms page exists for adding to email

[kimcoleman]

Description

There are edge cases where the terms link is a page that doesn't exist, but the logic in the email template still tried to add it as a link.

How has this been tested?

Locally.

Screenshots

Fixes:

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.