`jsonargparse==4.42.0` causing `test_config_help` failure

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`jsonargparse==4.42.0` is causing `test_config_help` failure while it passes untill `4.41.0`

```python

(litgpt) ⚡ fix/tests_config_hub ~/litgpt uv pip install jsonargparse==4.41.0
(litgpt) ⚡ fix/tests_config_hub ~/litgpt pytest tests/test_config_hub.py::test_config_help
============================================ test session starts =============================================
platform linux -- Python 3.12.11, pytest-8.4.2, pluggy-1.6.0
benchmark: 5.1.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /teamspace/studios/this_studio/litgpt
configfile: pyproject.toml
plugins: benchmark-5.1.0, anyio-4.11.0, timeout-2.4.0, rerunfailures-16.1, dependency-0.6.0
collected 46 items

tests/test_config_hub.py .............................................. [100%]

======================================= 46 passed, 6 warnings in 9.67s =======================================
```

```python
(litgpt) ⚡ fix/tests_config_hub ~/litgpt uv pip install jsonargparse==4.42.0
(litgpt) ⚡ fix/tests_config_hub ~/litgpt pytest tests/test_config_hub.py::test_config_help
.....
error: Parser key "model_config":
Does not validate against any of the Union subtypes
Subtypes: [, ]
Errors:
- Expected a
- Problem with given class_path 'litgpt.Config':
__args__
Given value type:
Given value: {'name': 'stories15M', 'hf_config': {}, 'scale_embeddings': False, 'block_size': 256, 'padded_vocab_size': 32000, 'n_layer': 6, 'n_head': 6, 'n_query_groups': 6, 'n_embd': 288, 'head_size': 48, 'rotary_percentage': 1.0, 'parallel_residual': False, 'bias': False, 'norm_class_name': 'RMSNorm', 'mlp_class_name': 'LLaMAMLP', 'intermediate_size': 768}
========================================== short test summary info ===========================================
FAILED tests/test_config_hub.py::test_config_help[litgpt/pretrain.py-pretrain/tinystories.yaml] - SystemExit: 2
FAILED tests/test_config_hub.py::test_config_help[litgpt/pretrain.py-https://raw.githubusercontent.com/Lightning-AI/litgpt/main/config_hub/pretrain/tinystorie

Start with tests/test_config_hub.py::test_config_help and reproduce the failure using jsonargparse 4.42.0, then compare it with 4.41.0. The work is done when the test_config_help cases pass with jsonargparse 4.42.0.

Written by the indexing model from the issue text.