### 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
user.create_homedir(user.home)
                user.chown_homedir(info[2], info[3], user.home)
            result['changed'] = True
        
        if user.sshkeygen:
            
            (rc, out, err) = user.ssh_key_gen()
            if rc is not None and rc != 0:
                module.fail_json(name=user.name, msg=err, rc=rc)
            if rc == 0:
                result['changed'] = True
            (rc, out, err) = user.ssh_key_fingerprint()
            if rc == 0:
                result['ssh_fingerprint'] = out.strip()
            else:
                result['ssh_fingerprint'] = err.strip()
            result['ssh_key_file'] = user.get_ssh_key_path()
            result['ssh_public_key'] = user.get_ssh_public_key()
        (rc, out, err) = user.set_password_expire()
        if rc is None:
            pass  
        else:
            if rc != 0:
                module.fail_json(name=user.name, msg=err, rc=rc)
            else:
                result['changed'] = True
    module.exit_json(**result)
if __name__ == '__main__':
    main()