### 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
- name: Remove the user 'johnd'
  ansible.builtin.user:
    name: johnd
    state: absent
    remove: yes
- name: Create a 2048-bit SSH key for user jsmith in ~jsmith/.ssh/id_rsa
  ansible.builtin.user:
    name: jsmith
    generate_ssh_key: yes
    ssh_key_bits: 2048
    ssh_key_file: .ssh/id_rsa
- name: Added a consultant whose account you want to expire
  ansible.builtin.user:
    name: james18
    shell: /bin/zsh
    groups: developers
    expires: 1422403387
- name: Starting at Ansible 2.6, modify user, remove expiry time
  ansible.builtin.user:
    name: james18
    expires: -1
- name: Set maximum expiration date for password
  ansible.builtin.user:
    name: ram19
    password_expire_max: 10
- name: Set minimum expiration date for password
  ansible.builtin.user:
    name: pushkar15
    password_expire_min: 5
'''