### 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
self.home = module.params['home']
        self.expires = None
        self.password_lock = module.params['password_lock']
        self.groups = None
        self.local = module.params['local']
        self.profile = module.params['profile']
        self.authorization = module.params['authorization']
        self.role = module.params['role']
        self.password_expire_max = module.params['password_expire_max']
        self.password_expire_min = module.params['password_expire_min']
        self.umask = module.params['umask']
        if self.umask is not None and self.local:
            module.fail_json(msg="'umask' can not be used with 'local'")
        if module.params['groups'] is not None:
            self.groups = ','.join(module.params['groups'])
        if module.params['expires'] is not None:
            try:
                self.expires = time.gmtime(module.params['expires'])
            except Exception as e:
                module.fail_json(msg="Invalid value for 'expires' %s: %s" % (self.expires, to_native(e)))
        if module.params['ssh_key_file'] is not None:
            self.ssh_file = module.params['ssh_key_file']
        else:
            self.ssh_file = os.path.join('.ssh', 'id_%s' % self.ssh_type)
        if self.groups is None and self.append:
            
            
            module.warn("'append' is set, but no 'groups' are specified. Use 'groups' for appending new groups."
                        "This will change to an error in Ansible 2.14.")
    def check_password_encrypted(self):