require target contact when creating an activity

when creating a new activity within a contact record, it's possible to delete the prepopulated value for the target/with contact field and save the contact with no target. that can happen if someone accidentally clicks backspace on form load, or misunderstands the field, or whatever. but regardless – i don't think we should allow an orphan activity record to be created like that. we should add a form rule to prevent saving.

are there instances where it is valid to have an activity without a target contact? do we need to make the form rule conditioned on context?