Invalid memberships when using the Restrict this page functionality

### Reproduction Steps

1. Create one or more memberships
2. Create a new page or edit an existing page
3. Turn on "Restrict this page" in the menu on the right
4. Select one or more memberships in the select box
5. Use the black "X" to remove memberships
6. Save
7. Select one membership
8. Save again
9. Refresh the page

### Expected Behavior

Only the one membership is shown

### Actual Behavior

Sometimes "None" shows as an option which then turns into " (ID#)" in the listing as a "phantom" membership. This can cause problems with restricting the access or showing an appropriate message when someone tries to access it who doesn't have access.

https://github.com/user-attachments/assets/8a31a0e1-aaec-433f-90b3-83f6d7fef4ab

### Tech Notes

It looks like these entries are saved as `""` (empty string) in the array of restricted IDs.

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