[3.13] gh-130292: Allow for empty simulator list when running iOS testbed (GH-130388) (#130532)

Adds error handling when there are no pre-existing test simulators.
(cherry picked from commit 99088ab081279329b8362e1c24533fa0be303e6f)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>