helix: don't ignore extraConfig when no other settings (#7302)

Fix so that you can still generate `programs.helix.extraConfig` without
using `programs.helix.settings`. Useful particularly in the case of
`extraConfig = lib.readFile ./config.toml;`.

modules/programs/helix.nix

@@ -229,12 +229,19 @@ in
 
     xdg.configFile =
       let
-        settings = {
+        settings =
-          "helix/config.toml" = mkIf (cfg.settings != { }) {
+          let
+            hasSettings = cfg.settings != { };
+            hasExtraConfig = cfg.extraConfig != null;
+          in
+          {
+            "helix/config.toml" = mkIf (hasSettings || hasExtraConfig) {
               source =
                 let
                   configFile = tomlFormat.generate "config.toml" cfg.settings;
-                extraConfigFile = pkgs.writeText "extra-config.toml" ("\n" + cfg.extraConfig);
+                  extraConfigFile = pkgs.writeText "extra-config.toml" (
+                    lib.optionalString hasSettings "\n" + cfg.extraConfig
+                  );
                 in
                 pkgs.runCommand "helix-config.toml" { } ''
                   cat ${configFile} ${extraConfigFile} >> $out

tests/modules/programs/helix/default.nix

@@ -1 +1,4 @@
-{ helix-example-settings = ./example-settings.nix; }
+{
+  helix-example-settings = ./example-settings.nix;
+  helix-only-extraconfig = ./only-extraconfig.nix;
+}

tests/modules/programs/helix/only-extraconfig-expected.toml

@@ -0,0 +1,2 @@
+[editor]
+auto-pairs = false

tests/modules/programs/helix/only-extraconfig.nix

@@ -0,0 +1,16 @@
+{ config, ... }:
+{
+  programs.helix = {
+    enable = true;
+    extraConfig = ''
+      [editor]
+      auto-pairs = false
+    '';
+  };
+
+  nmt.script = ''
+    assertFileContent \
+      home-files/.config/helix/config.toml \
+      ${./only-extraconfig-expected.toml}
+  '';
+}