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/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}
+    '';
+  };
+}