### 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.expires < time.gmtime(0):
                cmd.append('0')
            else:
                cmd.append(str(calendar.timegm(self.expires)))
        
        
        (rc, out, err) = self.execute_command(cmd)
        if rc is not None and rc != 0:
            self.module.fail_json(name=self.name, msg=err, rc=rc)
        
        if self.password is not None:
            cmd = [
                self.module.get_bin_path('chpass', True),
                '-p',
                self.password,
                self.name
            ]
            _rc, _out, _err = self.execute_command(cmd)
            if rc is None:
                rc = _rc
            out += _out
            err += _err
        
        _rc, _out, _err = self._handle_lock()
        if rc is None:
            rc = _rc
        out += _out
        err += _err
        return (rc, out, err)
    def modify_user(self):