bios: Handle empty pttype from lsblk output

zram, sr0 (CD/DVD) and LUKS devices generally don't use partitions, thus
don't have a partition table and don't have a partition type so this
field may be null.

Fixes: coreos#739

src/bios.rs

@@ -16,7 +16,7 @@ pub(crate) const GRUB_BIN: &str = "usr/sbin/grub2-install";
 #[derive(Serialize, Deserialize, Debug)]
 struct BlockDevice {
     path: String,
-    pttype: String,
+    pttype: Option<String>,
     parttypename: Option<String>,
 }
 
@@ -118,7 +118,7 @@ impl Bios {
         // Find the device with the parttypename "BIOS boot"
         for device in devices.blockdevices {
             if let Some(parttypename) = &device.parttypename {
-                if parttypename == "BIOS boot" && device.pttype == "gpt" {
+                if parttypename == "BIOS boot" && device.pttype == Some("gpt") {
                     return Ok(Some(device.path));
                 }
             }