[mount] If no btrfs layout at all, use something reasonable

This is related to https://invent.kde.org/neon/neon/calamares-settings/-/merge_requests/1
which adds .. the default things from the example configuration to the
configuration file KDE neon ships. The default layout doesn't add
any subvolumes at all, which seems to be non-functional.

If nothing is configured, complain and use /@ as the lone subvolume.

src/modules/mount/main.py

@@ -74,7 +74,14 @@ def mount_partition(root_mount_point, partition, partitions):
     # Special handling for btrfs subvolumes. Create the subvolumes listed in mount.conf
     if fstype == "btrfs" and partition["mountPoint"] == '/':
         # Root has been mounted to btrfs volume -> create subvolumes from configuration
-        btrfs_subvolumes = libcalamares.job.configuration.get("btrfsSubvolumes") or []
+        btrfs_subvolumes = libcalamares.job.configuration.get("btrfsSubvolumes", None)
+        # Warn if there's no configuration at all, and empty configurations are
+        # replaced by a simple root-only layout.
+        if btrfs_subvolumes is None:
+            libcalamares.utils.warning("No configuration for btrfsSubvolumes")
+        if not btrfs_subvolumes:
+            btrfs_subvolumes = [ dict(mountPoint="/", subvolume="/@") ]
+
         subvolume_mountpoints = [d['mountPoint'] for d in btrfs_subvolumes]
         # Check if listed mountpoints besides / are already present and don't create subvolume for those
         for p in partitions: