storaged: btrfs: don't break on a btrfs mount without subvol

We always assume fstab_subvol to be truthy while when fstab contains a
btrfs mount without a subvol option it is not and we should just show
the root subvolume.

In an ideal case we would call GetDefaultSubvolume when the filesystem
is mounted and then show the subvolume name instead. I'll leave this for
a follow up.

pkg/storaged/btrfs/volume.jsx

@@ -173,13 +173,14 @@ function make_btrfs_subvolume_pages(parent, volume) {
 
                 const fstab_subvol = parse_subvol_from_options(decode_filename(opts.v));
 
-                if (fstab_subvol && fstab_subvol.pathname === "/") {
+                if (fstab_subvol === null)
+                    continue;
+
+                if (fstab_subvol.pathname === "/")
                     has_root = true;
-                }
 
-                if (fstab_subvol.pathname) {
+                if (fstab_subvol.pathname)
                     make_btrfs_subvolume_page(parent, volume, fstab_subvol);
-                }
             }
         }
 

test/verify/check-storage-btrfs

@@ -603,6 +603,25 @@ ExecStart=/usr/bin/sleep infinity
         self.confirm()
         b.wait_in_text(self.card_row("Storage", location=mount_point), "btrfs subvolume")
 
+    def testNoSubvolMount(self):
+        m = self.machine
+        b = self.browser
+
+        disk = self.add_ram_disk(size=128)
+        mount_point = "/run/butter"
+
+        m.execute(f"""
+            mkfs.btrfs -L butter {disk}
+            mkdir -p {mount_point}
+            mount {disk} {mount_point}
+            echo '{disk} {mount_point} auto defaults 0 0' >> /etc/fstab
+        """)
+
+        self.login_and_go("/storage")
+
+        self.click_card_row("Storage", name="butter")
+        b.wait_visible(self.card_row("btrfs subvolumes", name="/"))
+
     def testNothingMounted(self):
         m = self.machine
         b = self.browser