types btrfs: fix format without swap or subvolumes

example/btrfs-only-root-subvolume.nix

@@ -0,0 +1,37 @@
+{
+  disko.devices = {
+    disk = {
+      vdb = {
+        type = "disk";
+        device = "/dev/disk/by-diskseq/1";
+        content = {
+          type = "gpt";
+          partitions = {
+            ESP = {
+              priority = 1;
+              name = "ESP";
+              start = "1M";
+              end = "128M";
+              type = "EF00";
+              content = {
+                type = "filesystem";
+                format = "vfat";
+                mountpoint = "/boot";
+              };
+            };
+            root = {
+              size = "100%";
+              content = {
+                type = "btrfs";
+                extraArgs = [ "-f" ]; # Override existing partition
+                mountpoint = "/";
+                mountOptions = [ "compress=zstd" "noatime" ];
+              };
+            };
+          };
+        };
+      };
+    };
+  };
+}
+

lib/types/btrfs.nix

@@ -120,6 +120,7 @@ in
         if ! (blkid '${config.device}' -o export | grep -q '^TYPE='); then
           mkfs.btrfs "${config.device}" ${toString config.extraArgs}
         fi
+        ${lib.optionalString (config.swap != {} || config.subvolumes != {}) ''
           if (blkid "${config.device}" -o export | grep -q '^TYPE=btrfs$'); then
             ${lib.optionalString (config.swap != {}) ''
               (
@@ -143,6 +144,7 @@ in
               )
             '') (lib.attrValues config.subvolumes)}
           fi
+        ''}
       '';
     };
     _mount = diskoLib.mkMountOption {

tests/btrfs-only-root-subvolume.nix

@@ -0,0 +1,11 @@
+{ pkgs ? import <nixpkgs> { }
+, diskoLib ? pkgs.callPackage ../lib { }
+}:
+diskoLib.testLib.makeDiskoTest {
+  inherit pkgs;
+  name = "btrfs-only-root-subvolume";
+  disko-config = ../example/btrfs-only-root-subvolume.nix;
+  extraTestScript = ''
+    machine.succeed("btrfs subvolume list /");
+  '';
+}