fix(tasks): use optionalString instead of mkIf for env.DEVENV_TASKS

mkIf on a freeform submodule attribute registers the key but leaves no
value when the condition is false, causing "was accessed but has no
value defined" during derivation evaluation. optionalString returns ""
instead, which is always a valid value.

Fixes #2532

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: domenkozar

src/modules/tasks.nix

@@ -381,7 +381,7 @@ in
     # Only set DEVENV_TASKS env var for flakes integration or pre-2.0 CLI,
     # where devenv-tasks is invoked via shell hooks and needs the env var.
     # For 2.0+, DEVENV_TASK_FILE is sufficient and avoids bloating the environment.
-    env.DEVENV_TASKS = lib.mkIf (config.devenv.cli.version == null || lib.versionOlder config.devenv.cli.version "2.0") (builtins.toJSON tasksJSON);
+    env.DEVENV_TASKS = lib.optionalString (config.devenv.cli.version == null || lib.versionOlder config.devenv.cli.version "2.0") (builtins.toJSON tasksJSON);
     env.DEVENV_TASK_FILE = config.task.config;
     task.config = (pkgs.formats.json { }).generate "tasks.json" tasksJSON;