### 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
new_cls = get_platform_subclass(User)
        return super(cls, new_cls).__new__(new_cls)
    def __init__(self, module):
        self.module = module
        self.state = module.params['state']
        self.name = module.params['name']
        self.uid = module.params['uid']
        self.hidden = module.params['hidden']
        self.non_unique = module.params['non_unique']
        self.seuser = module.params['seuser']
        self.group = module.params['group']
        self.comment = module.params['comment']
        self.shell = module.params['shell']
        self.password = module.params['password']
        self.force = module.params['force']
        self.remove = module.params['remove']
        self.create_home = module.params['create_home']
        self.move_home = module.params['move_home']
        self.skeleton = module.params['skeleton']
        self.system = module.params['system']
        self.login_class = module.params['login_class']
        self.append = module.params['append']
        self.sshkeygen = module.params['generate_ssh_key']
        self.ssh_bits = module.params['ssh_key_bits']
        self.ssh_type = module.params['ssh_key_type']
        self.ssh_comment = module.params['ssh_key_comment']
        self.ssh_passphrase = module.params['ssh_key_passphrase']
        self.update_password = module.params['update_password']