### 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
description: Whether or not the account is a system account.
  returned: When I(system) is passed to the module and the account does not exist
  type: bool
  sample: True
uid:
  description: User ID of the user account.
  returned: When I(uid) is passed to the module
  type: int
  sample: 1044
'''
import ctypes.util
import grp
import calendar
import os
import re
import pty
import pwd
import select
import shutil
import socket
import subprocess
import time
import math
from ansible.module_utils import distro
from ansible.module_utils.common.text.converters import to_bytes, to_native, to_text
from ansible.module_utils.basic import AnsibleModule
from ansible.module_utils.common.locale import get_best_parsable_locale
from ansible.module_utils.common.sys_info import get_platform_subclass
import ansible.module_utils.compat.typing as t
class StructSpwdType(ctypes.Structure):