### 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
cmd.append(self.login_class)
        if self.update_password == 'always' and self.password is not None and info[1] != self.password:
            cmd.append('-p')
            cmd.append(self.password)
        if self.password_lock and not info[1].startswith('*LOCKED*'):
            cmd.append('-C yes')
        elif self.password_lock is False and info[1].startswith('*LOCKED*'):
            cmd.append('-C no')
        
        if len(cmd) == 1:
            return (None, '', '')
        cmd.append(self.name)
        return self.execute_command(cmd)
class SunOS(User):
    """
    This is a SunOS User manipulation class - The main difference between
    this class and the generic user class is that Solaris-type distros
    don't support the concept of a "system" account and we need to
    edit the /etc/shadow file manually to set a password. (Ugh)
    This overrides the following methods from the generic class:-
      - create_user()
      - remove_user()
      - modify_user()
      - user_info()
    """
    platform = 'SunOS'
    distribution = None
    SHADOWFILE = '/etc/shadow'
    USER_ATTR = '/etc/user_attr'
    def get_password_defaults(self):