Manage PIDs more carefully in DaemonHandle

Summary:
Ref T13555. Although these callsites may not actually impact anything, it's possible for an active handle to have no PID (e.g., if the subprocess failed to start).

Handle these cases more carefully.

Test Plan: Started daemons, saw them run fine. See also next change.

Maniphest Tasks: T13555

Differential Revision: https://secure.phabricator.com/D21424