Fix sparse image regression introduced by !2177
Deviceinfo variables are empty strings by default if they are unset
in the deviceinfo file, so the is not None check currently enables
the sm_sparse_image_tool for all existing devices using sparse format.
Simplify the check to convert the string to a bool. Empty strings evaluate to False so this should work as originally intended.
Should fix the following 38+ failing bpo image builds (more will likely fail):
