Error parsing PEP 735 dependent dependency groups

```python
import toml

toml.loads(
    """
[dependency-groups]
foo = []
bar = ["foo", { include-group = "foo" }, "bar"]
baz = ["foo", { include-group = "foo" }]
"""
)
```

`python bug.py`:
```
...
  File "C:\Git\project\.venv\Lib\site-packages\toml\decoder.py", line 1002, in load_array
    a[b] = a[b] + ',' + a[b + 1]
                        ~^^^^^^^
IndexError: list index out of range
```