### 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
if rc is not None and rc != 0:
            module.fail_json(name=user.name, msg=err, rc=rc)
        if user.password is not None:
            result['password'] = 'NOT_LOGGING_PASSWORD'
    if rc is None:
        result['changed'] = False
    else:
        result['changed'] = True
    if out:
        result['stdout'] = out
    if err:
        result['stderr'] = err
    if user.user_exists() and user.state == 'present':
        info = user.user_info()
        if info is False:
            result['msg'] = "failed to look up user name: %s" % user.name
            result['failed'] = True
        result['uid'] = info[2]
        result['group'] = info[3]
        result['comment'] = info[4]
        result['home'] = info[5]
        result['shell'] = info[6]
        if user.groups is not None:
            result['groups'] = user.groups
        
        info = user.user_info()
        if user.home is None:
            user.home = info[5]
        if not os.path.exists(user.home) and user.create_home:
            if not module.check_mode: