[services] Backup - Fix terminate callback in pull backup reactor

Summary: We have to check whether the get reactor is initialized in the pull backup reactor before checking its status. The initialization of this reactor happens in the writeResponse method so if we have an error in the initialize method, terminate's going to be called with get reactor uninitialized and that will lead to a crash.

Test Plan: make a request for pulling a backup without passing a backup id (missing any fields would trigger this I think).

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4223