fcitx5-rime: ensure 'default.yaml' exists to prevent startup issues (#416105)

pkgs/tools/inputmethods/fcitx5/fcitx5-rime.nix

···
   rimeDataDrv = symlinkJoin {
     name = "fcitx5-rime-data";
     paths = rimeDataPkgs;
-    postBuild = "mkdir -p $out/share/rime-data";
+    postBuild = ''
+      mkdir -p $out/share/rime-data
+
+      # Ensure default.yaml exists
+      [ -e "$out/share/rime-data/default.yaml" ] || touch "$out/share/rime-data/default.yaml"
+    '';
   };
 
   postInstall = ''