wezterm: don't create config if extraConfig is empty, and don't create one by default (#6908)

Tests updated to check the file wasn't created when no config is
provided.

modules/programs/wezterm.nix

@@ -9,6 +9,7 @@ let
   inherit (lib)
     literalExpression
     mkIf
+    mkMerge
     mkEnableOption
     mkOption
     types
@@ -35,9 +36,7 @@ in
 
     extraConfig = mkOption {
       type = types.lines;
-      default = ''
-        return {}
-      '';
+      default = "";
       example = literalExpression ''
         -- Your lua code / config here
         local mylib = require 'mylib';
@@ -100,8 +99,8 @@ in
   config = mkIf cfg.enable {
     home.packages = [ cfg.package ];
 
-    xdg.configFile =
-      {
+    xdg.configFile = mkMerge [
+      (mkIf (cfg.extraConfig != "") {
         "wezterm/wezterm.lua".text = ''
           -- Generated by Home Manager.
           -- See https://wezfurlong.org/wezterm/
@@ -110,13 +109,14 @@ in
 
           ${cfg.extraConfig}
         '';
-      }
-      // lib.mapAttrs' (
+      })
+      (lib.mapAttrs' (
         name: value:
         lib.nameValuePair "wezterm/colors/${name}.toml" {
           source = tomlFormat.generate "${name}.toml" { colors = value; };
         }
-      ) cfg.colorSchemes;
+      ) cfg.colorSchemes)
+    ];
 
     programs.bash.initExtra = mkIf cfg.enableBashIntegration shellIntegrationStr;
     programs.zsh.initContent = mkIf cfg.enableZshIntegration shellIntegrationStr;