### 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
type: str
    state:
        description:
            - Whether the account should exist or not, taking action if the state is different from what is stated.
            - See this L(FAQ entry,https://docs.ansible.com/ansible/latest/reference_appendices/faq.html#running-on-macos-as-a-target)
              for additional requirements when removing users on macOS systems.
        type: str
        choices: [ absent, present ]
        default: present
    create_home:
        description:
            - Unless set to C(false), a home directory will be made for the user
              when the account is created or if the home directory does not exist.
            - Changed from C(createhome) to C(create_home) in Ansible 2.5.
        type: bool
        default: yes
        aliases: [ createhome ]
    move_home:
        description:
            - "If set to C(true) when used with C(home: ), attempt to move the user's old home
              directory to the specified directory if it isn't there already and the old home exists."
        type: bool
        default: no
    system:
        description:
            - When creating an account C(state=present), setting this to C(true) makes the user a system account.
            - This setting cannot be changed on existing users.
        type: bool
        default: no
    force: