os.walk() silent about non-directory

`os.walk()` silently produces an empty iterator if the argument  is not a path to directory:

```pycon
>>> import os
>>> list(os.walk('./non-existing'))
[]
>>> list(os.walk('./python'))
[]
>>> list(os.walk('/dev/null'))
[]
```

It can causes bugs like #99203, when a function produces some incorrect result instead of raising an exception.
