### 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
ssh_key_passphrase=dict(type='str', no_log=True),
            update_password=dict(type='str', default='always', choices=['always', 'on_create'], no_log=False),
            expires=dict(type='float'),
            password_lock=dict(type='bool', no_log=False),
            local=dict(type='bool'),
            profile=dict(type='str'),
            authorization=dict(type='str'),
            role=dict(type='str'),
            umask=dict(type='str'),
        ),
        supports_check_mode=True,
    )
    user = User(module)
    user.check_password_encrypted()
    module.debug('User instantiated - platform %s' % user.platform)
    if user.distribution:
        module.debug('User instantiated - distribution %s' % user.distribution)
    rc = None
    out = ''
    err = ''
    result = {}
    result['name'] = user.name
    result['state'] = user.state
    if user.state == 'absent':
        if user.user_exists():
            if module.check_mode:
                module.exit_json(changed=True)
            (rc, out, err) = user.remove_user()
            if rc != 0:
                module.fail_json(name=user.name, msg=err, rc=rc)