Single-threaded Web exports are broken

[adamscott]

Tested versions

• Bisected to Make use of languages' thread enter/exit more correct #96760
  • EDIT: as the PR has two commits, it actually bisects to 2d1dd41

System information

Godot v4.4.dev (2c136e6) - macOS 14.6.1 - Metal (Forward+) - integrated Apple M3 Max (Apple9) - Apple M3 Max (16 Threads)

Issue description

This errors shows up on non-threaded web exports.

ERROR: Can't add a group task because the WorkerThreadPool is either not initialized yet or already terminated. [tmp_js_export.js:474:18](http://localhost:8060/tmp_js_export.js)
   at: _add_group_task (core/object/worker_thread_pool.cpp:546) [tmp_js_export.js:474:18](http://localhost:8060/tmp_js_export.js)

Steps to reproduce

1. Export a web game without thread support
2. See that there's a flood of the described error

Minimal reproduction project (MRP)

n/a

[adamscott]

@RandomShaper From a quick test, it seems to be related to this new check:

godot/core/object/worker_thread_pool.cpp

Line 546 in a75bace

ERR_FAIL_COND_V_MSG(threads.is_empty(), INVALID_TASK_ID, "Can't add a group task because the WorkerThreadPool is either not initialized yet or already terminated.");

Because before the added check, single-threaded builds would call WorkerThreadPool::_add_group_task() without any (apparent) issue.