### 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.local:
                        lexpires = -1
                    else:
                        cmd.append('-e')
                        cmd.append('')
            else:
                
                current_expire_date = time.gmtime(current_expires * 86400)
                
                if current_expires < 0 or current_expire_date[:3] != self.expires[:3]:
                    if self.local:
                        
                        lexpires = int(math.floor(self.module.params['expires'])) // 86400
                    else:
                        cmd.append('-e')
                        cmd.append(time.strftime(self.DATE_FORMAT, self.expires))
        
        if self.password_lock and not info[1].startswith('!'):
            cmd.append('-L')
        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 info[1].lstrip('!') != self.password.lstrip('!'):
            
            cmd = [c for c in cmd if c not in ['-U', '-L']]
            cmd.append('-p')
            if self.password_lock:
                
                cmd.append('!%s' % self.password)
            else: