prompt_toolkit.shortcuts.confirm doesn't always return bool

`prompt_toolkit.shortcuts.confirm` doesn't always return `bool`. When pressing just enter it returns a `str`.

For example:
```python
from prompt_toolkit.shortcuts import confirm

result = confirm()

print(f"Result|{result}|")
print(type(result))
```
Produces:
```
Confirm? (y/n)  ### I press `Enter` here ###
Result||
<class 'str'>
```
This is on Windows, using prompt_toolkit 3.0.52 if it matters.