nullcontext: untyped or Any enter_result type is ignored and inferred as None on __enter__

**Describe the bug**
The `nullcontext` context manager from stdlib lets to pass arbitrary value to be returned on `__enter__`. It's named `enter_result`.
If the value passed is untyped or `Any`, pyright infers return type of `__enter__` as `None`. But should preserve untyped/`Any` type.

**Code or Screenshots**
```python
from contextlib import nullcontext
from typing import Any, revela_type

# also with: def test(items) -> None:
def test(items: Any) -> None:
with nullcontext(items) as _items:
reveal_type(_items)
for item in _items:
print(item)

if __name__ == '__main__':
test([1, 2, 3])
```

**Expected behavior**
Revealed type is Any, no errors

**Observed behavior**
```sh
$ pyright ./test.py
test.py
test.py:9:21 - information: Type of "_items" is "None"
test.py:10:21 - error: "None" is not iterable
  "__iter__" method not defined (reportGeneralTypeIssues)
1 error, 0 warnings, 1 information
```

**VS Code extension or command-line**
CLI pyright 1.1.408

Start by reproducing the report with the shown test.py snippet and the CLI command `pyright ./test.py`. Trace how `contextlib.nullcontext` is typed when its `enter_result` argument is untyped or Any, then verify that `_items` is revealed as Any and the iteration produces no error.

Written by the indexing model from the issue text.