### 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
return (rc, _out, _err)
        (rc, _out, _err) = self._change_user_password()
        out += _out
        err += _err
        self._update_system_user()
        
        
        if self.groups:
            (rc, _out, _err, changed) = self._modify_group()
            out += _out
            err += _err
        return (rc, out, err)
    def modify_user(self):
        changed = None
        out = ''
        err = ''
        if self.group:
            self._make_group_numerical()
        for field in self.fields:
            if field[0] in self.__dict__ and self.__dict__[field[0]]:
                current = self._get_user_property(field[1])
                if current is None or current != to_text(self.__dict__[field[0]]):
                    cmd = self._get_dscl()
                    cmd += ['-create', '/Users/%s' % self.name, field[1], self.__dict__[field[0]]]
                    (rc, _out, _err) = self.execute_command(cmd)
                    if rc != 0:
                        self.module.fail_json(
                            msg='Cannot update property "%s" for user "%s".'
                                % (field[0], self.name), err=err, out=out, rc=rc)
                    changed = rc