If the Terms and Conditions page is invalid, the link during checkout is incorrect / broken

### Reproduction Steps

1. Set a "Terms and Conditions Page" in _LifterLMS > Settings_, Accounts tab.
2. Move the selected page to Trash (in Pages)

### Expected Behavior

Show a warning that the Terms and Conditions page is invalid

### Actual Behavior

There is no warning, the link is broken in checkout (if in Trash) or does not appear at all if the page is deleted permanently.


This issue has be recreated:
+ [ ] Locally
+ [ ] On a staging site
+ [ ] On a production website
+ [ ] With only LifterLMS and a default theme

### Browser, Device, and Operating System Information

+ Browser name and version
+ Operating System name and version
+ Device name and version (if applicable)