### 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
result['force'] = user.force
            result['remove'] = user.remove
    elif user.state == 'present':
        if not user.user_exists():
            if module.check_mode:
                module.exit_json(changed=True)
            
            
            path_needs_parents = False
            if user.home and user.create_home:
                parent = os.path.dirname(user.home)
                if not os.path.isdir(parent):
                    path_needs_parents = True
            (rc, out, err) = user.create_user()
            
            
            if path_needs_parents:
                info = user.user_info()
                if info is not False:
                    user.chown_homedir(info[2], info[3], user.home)
            if module.check_mode:
                result['system'] = user.name
            else:
                result['system'] = user.system
                result['create_home'] = user.create_home
        else:
            
            (rc, out, err) = user.modify_user()
            result['append'] = user.append
            result['move_home'] = user.move_home