pmb.install._install: Use bool(...) when figuring out whether to use cgpt (MR 2464)

Otherwise, if pmb.parse.deviceinfo().cgpt_kpart is an empty string, for
example, we end up passing an empty string to get_partition_layout()
instead of False.

pmb/install/_install.py

@@ -859,7 +859,7 @@ def install_system_image(
     pmb.helpers.mount.umount_all(chroot.path)
     (size_boot, size_root) = get_subpartitions_size(chroot)
     layout = get_partition_layout(
-        size_reserve, pmb.parse.deviceinfo().cgpt_kpart and args.install_cgpt
+        size_reserve, bool(pmb.parse.deviceinfo().cgpt_kpart and args.install_cgpt)
     )
     if not args.rsync:
         pmb.install.blockdevice.create(args, size_boot, size_root, size_reserve, split, disk)