### 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
non_unique=dict(type='bool', default=False),
            group=dict(type='str'),
            groups=dict(type='list', elements='str'),
            comment=dict(type='str'),
            home=dict(type='path'),
            shell=dict(type='str'),
            password=dict(type='str', no_log=True),
            login_class=dict(type='str'),
            password_expire_max=dict(type='int', no_log=False),
            password_expire_min=dict(type='int', no_log=False),
            
            hidden=dict(type='bool'),
            
            seuser=dict(type='str'),
            
            force=dict(type='bool', default=False),
            remove=dict(type='bool', default=False),
            
            create_home=dict(type='bool', default=True, aliases=['createhome']),
            skeleton=dict(type='str'),
            system=dict(type='bool', default=False),
            
            move_home=dict(type='bool', default=False),
            append=dict(type='bool', default=False),
            
            generate_ssh_key=dict(type='bool'),
            ssh_key_bits=dict(type='int', default=ssh_defaults['bits']),
            ssh_key_type=dict(type='str', default=ssh_defaults['type']),
            ssh_key_file=dict(type='path'),
            ssh_key_comment=dict(type='str', default=ssh_defaults['comment']),