### 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
if self.shell is not None and info[6] != self.shell:
            cmd.append('-s')
            cmd.append(self.shell)
        if self.login_class is not None:
            
            user_login_class = None
            userinfo_cmd = [self.module.get_bin_path('userinfo', True), self.name]
            (rc, out, err) = self.execute_command(userinfo_cmd, obey_checkmode=False)
            for line in out.splitlines():
                tokens = line.split()
                if tokens[0] == 'class' and len(tokens) == 2:
                    user_login_class = tokens[1]
            
            if self.login_class != user_login_class:
                cmd.append('-L')
                cmd.append(self.login_class)
        if self.password_lock and not info[1].startswith('*'):
            cmd.append('-Z')
        elif self.password_lock is False and info[1].startswith('*'):
            cmd.append('-U')
        if self.update_password == 'always' and self.password is not None \
                and self.password != '*' and info[1] != self.password:
            cmd.append('-p')
            cmd.append(self.password)
        
        if len(cmd) == 1:
            return (None, '', '')
        cmd.append(self.name)
        return self.execute_command(cmd)
class NetBSDUser(User):