[mender-convert-modify] Allow images with no initrd

As probe_initrd_in_boot_and_root indicates, this is not always required.
Adding the check for the ln not to throw an error.

Changelog: None

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

mender-convert-modify

@@ -143,7 +143,9 @@ initrd_imagetype=initrd
 EOF
 
   run_and_log_cmd "ln -s ${kernel_imagetype} work/rootfs/boot/kernel"
+  if [ "${initrd_imagetype}" != "" ]; then
     run_and_log_cmd "ln -s ${initrd_imagetype} work/rootfs/boot/initrd"
+  fi
 
   # For partuuid support grub.cfg expects dedicated variables to be added
   if [ "${MENDER_ENABLE_PARTUUID}" == "y" ]; then