CRM-21507 Fix error when creating case activities

CRM-21507 Fix error when creating case activities

Before: When adding a new activity to a case, an error message
"Activity Separation is a required field" appears and prevents the user
from submitting the form. There is no way to get around this error.

After: The user can create case activities without hitting this error
message. Also the error still displays as expected when creating
non-case activities that have multiple target contacts.

This changes fixes a regression caused by my fix for CRM-21419.