Add hard stops on empty batch edit sets

Summary:
Ref T8637. If a user tries to batch edit a list of tasks which can't be edited, we fall through to withIDs(array()), which can affect everything.

Explicitly stop batch editing if we don't have valid IDs or valid tasks.

The UI sort-of warns you that something is wrong, but this is ultimately a pretty severe UX issue. I'll fix the underlying Query in the next diff.

Test Plan: Tried to batch edit a list of tasks I didn't have permission to edit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: lloyd.oliver, epriestley

Maniphest Tasks: T8637

Differential Revision: https://secure.phabricator.com/D13388