### Summary

If you set `skeleton` to `/dev/null` in `user` module you get an error saying that it's not a directory. However it's perfectly fine to do `useradd --skel /dev/null`, so ansible shouldn't be limiting the functionality of the `useradd` command.

`useradd --skel /dev/null` will cause nothing to be added to the new home directoy. It used to work correctly in older versions of ansible and should be fixed in this version.

### Issue Type

Bug Report

### Component Name

user

### Ansible Version

```console
$ ansible --version
2.11.1
```


### Configuration

```console
$ ansible-config dump --only-changed
```


### OS / Environment

linux

### Steps to Reproduce

<!--- Paste example playbooks or commands between quotes below -->
```yaml (paste below)

```


### Expected Results

`/dev/null` should be allowed and result in no files being added to new home directory

### Actual Results

```console
It caused an error saying that /dev/null is not a directory
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
out += _out
                    err += _err
        if self.update_password == 'always' and self.password is not None:
            (rc, _out, _err) = self._change_user_password()
            out += _out
            err += _err
            changed = rc
        if self.groups:
            (rc, _out, _err, _changed) = self._modify_group()
            out += _out
            err += _err
            if _changed is True:
                changed = rc
        rc = self._update_system_user()
        if rc == 0:
            changed = rc
        return (changed, out, err)
class AIX(User):
    """
    This is a AIX User manipulation class.
    This overrides the following methods from the generic class:-
      - create_user()
      - remove_user()
      - modify_user()
      - parse_shadow_file()
    """
    platform = 'AIX'
    distribution = None
    SHADOWFILE = '/etc/security/passwd'
    def remove_user(self):