stylix/palette: coerce derivations to store paths

Coerce derivations to store paths to ensure non-null values are
stringified store paths with valid string contexts.

Fixes: 838df8b8ad ("stylix: improve `stylix.image` type (#1414)")

Co-authored-by: Matt Sturgeon <matt@sturgeon.me.uk>

stylix/palette.nix

@@ -28,10 +28,8 @@ in
     };
 
     image = lib.mkOption {
-      type = lib.types.nullOr lib.types.path;
       # Ensure the path is copied to the store
-      apply =
-        value: if value == null || lib.isDerivation value then value else "${value}";
+      type = with lib.types; nullOr (coercedTo path (src: "${src}") pathInStore);
       description = ''
         Wallpaper image.