UnsetType union types  lowering is incomplete

### Description

```python
from typing import assert_type
from msgspec import UnsetType


def f(s: str | UnsetType):
    if s:
        assert_type(s, str)  # this fails
```

`UNSET` is the only `UnsetType` value and `bool(UNSET) is False`. Therefore the if statement should narrow the type of `s` to `str`.

