spec: fix conversation batch spec for non-empty db

Change-Id: I71947c68f66e6a07b5172384217b5b068874886e
Reviewed-on: https://gerrit.instructure.com/28866
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>