fix: validation error for empty Batch Enrollment/Beta Tester to Quince

[DmytroAlipov]

This is a backport from the master.

Description

[Instructor Dashboard > Membership] Validation error for "empty" Batch Enrollment, Batch Beta Tester Addition fields is absent

Steps to Reproduce:

1. Go to the Instructor Dashboard > Membership
2. Leave input field for Batch Enrollment empty
3. Click on [Enroll] button

Another bug found:

• if you add invalid users to Batch Enrollment:

• if you add invalid users to Batch Beta Tester Addition:

Now everything works correctly:

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[mphilbrick211]

Hi @openedx/2u-fed-bom! Would someone be able to merge this for us? You recently merged the master.

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