### 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
'''Change password for SELF.NAME against SELF.PASSWORD.
        Please note that password must be cleartext.
        '''
        
        
        
        
        
        
        
        cmd = self._get_dscl()
        if self.password:
            cmd += ['-passwd', '/Users/%s' % self.name, self.password]
        else:
            cmd += ['-create', '/Users/%s' % self.name, 'Password', '*']
        (rc, out, err) = self.execute_command(cmd)
        if rc != 0:
            self.module.fail_json(msg='Error when changing password', err=err, out=out, rc=rc)
        return (rc, out, err)
    def _make_group_numerical(self):