kitty: fix diff.conf being generated regardless of user config

'cfg.diffConfig' is an implicit submodule with sub-options that carry
their own defaults, so it never actually equals `{ }` - the previous
`cfg.diffConfig != { }` check was always true. This caused
'kitty/diff.conf' to be written (and clobber any existing unmanaged file)
even when the user configured nothing under `programs.kitty.diffConfig`.

tests/modules/programs/kitty/default.nix

@@ -5,4 +5,5 @@
   kitty-example-mkOrder = ./example-mkOrder.nix;
   kitty-example-quickAccessTerminalConfig = ./example-quickAccessTerminalConfig.nix;
   kitty-auto-theme-files = ./auto-theme-files.nix;
+  kitty-example-diffConfig = ./example-diffConfig.nix;
 }

tests/modules/programs/kitty/example-diffConfig-expected.conf

@@ -0,0 +1,6 @@
+# Generated by Home Manager
+# See https://sw.kovidgoyal.net/kitty/kittens/diff/
+diff_cmd auto
+
+map j scroll_by 1
+

tests/modules/programs/kitty/example-diffConfig.nix

@@ -0,0 +1,21 @@
+{
+  config = {
+    programs.kitty = {
+      enable = true;
+
+      diffConfig = {
+        settings.diff_cmd = "auto";
+        keybindings = {
+          "j" = "scroll_by 1";
+        };
+      };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/kitty/diff.conf
+      assertFileContent \
+        home-files/.config/kitty/diff.conf \
+        ${./example-diffConfig-expected.conf}
+    '';
+  };
+}