Fix `DeleteCollaboration` transaction behaviour (#28886)

Fix `DeleteCollaboration` transaction behaviour (#28886)

The method can't be called with an outer transaction because if the user
is not a collaborator the outer transaction will be rolled back even if
the inner transaction uses the no-error path.

`has == 0` leads to `return nil` which cancels the transaction. A
standalone call of this method does nothing but if used with an outer
transaction, that will be canceled.