### 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 the next available uid. If system=True, then
        uid should be below of 500, if possible.
        '''
        cmd = self._get_dscl()
        cmd += ['-list', '/Users', 'UniqueID']
        (rc, out, err) = self.execute_command(cmd, obey_checkmode=False)
        if rc != 0:
            self.module.fail_json(
                msg="Unable to get the next available uid",
                rc=rc,
                out=out,
                err=err
            )
        max_uid = 0
        max_system_uid = 0
        for line in out.splitlines():
            current_uid = int(line.split(' ')[-1])
            if max_uid < current_uid:
                max_uid = current_uid
            if max_system_uid < current_uid and current_uid < 500:
                max_system_uid = current_uid
        if system and (0 < max_system_uid < 499):
            return max_system_uid + 1
        return max_uid + 1
    def _change_user_password(self):