### 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
cmd = self._get_dscl()
        cmd += ['-create', '/Users/%s' % self.name]
        (rc, out, err) = self.execute_command(cmd)
        if rc != 0:
            self.module.fail_json(msg='Cannot create user "%s".' % self.name, err=err, out=out, rc=rc)
        self._make_group_numerical()
        if self.uid is None:
            self.uid = str(self._get_next_uid(self.system))
        
        if self.create_home:
            if self.home is None:
                self.home = '/Users/%s' % self.name
            if not self.module.check_mode:
                if not os.path.exists(self.home):
                    os.makedirs(self.home)
                self.chown_homedir(int(self.uid), int(self.group), self.home)
        
        
        if not self.system and self.shell is None:
            self.shell = '/bin/bash'
        for field in self.fields:
            if field[0] in self.__dict__ and 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 add property "%s" to user "%s".' % (field[0], self.name), err=err, out=out, rc=rc)
                out += _out
                err += _err
                if rc != 0: