### 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:
            - Optionally specify the type of SSH key to generate.
            - Available SSH key types will depend on implementation
              present on target host.
        type: str
        default: rsa
        version_added: "0.9"
    ssh_key_file:
        description:
            - Optionally specify the SSH key filename.
            - If this is a relative filename then it will be relative to the user's home directory.
            - This parameter defaults to I(.ssh/id_rsa).
        type: path
        version_added: "0.9"
    ssh_key_comment:
        description:
            - Optionally define the comment for the SSH key.
        type: str
        default: ansible-generated on $HOSTNAME
        version_added: "0.9"
    ssh_key_passphrase:
        description:
            - Set a passphrase for the SSH key.
            - If no passphrase is provided, the SSH key will default to having no passphrase.
        type: str
        version_added: "0.9"
    update_password:
        description:
            - C(always) will update passwords if they differ.
            - C(on_create) will only set the password for newly created users.