Can't remove Membership setting when editing an access plan.

[ideadude]

Reproduction Steps

1. Edit a course.
2. Add an access plan.
3. Set the plan to require membership.
4. Choose a membership.
5. Save All Plans.
6. Reload the page.
7. Try to remove the membership by clicking the "x".
8. Save All Plans.
9. Re open the access plan and note the membership setting is still there.

This also happens for the the WooCommerce Product setting when using our integration plugin for WooCommerce. It may be happening for any dropdown or select2 type field that has a blank option.

Video of the error in action: https://share.zight.com/P8uzk2xp

Expected Behavior

The access plan should update to have the blank value. If a value is required, it should tell the user that the value is required before saving.

Actual Behavior

The plans save, but the membership value is not blanked out and so comes back with the last saved value.

This issue has be recreated:

• [x] Locally
• [x] On a staging site
• [ ] On a production website
• [x] With only LifterLMS and a default theme