### 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
default: no
        version_added: "2.4"
    profile:
        description:
            - Sets the profile of the user.
            - Does nothing when used with other platforms.
            - Can set multiple profiles using comma separation.
            - To delete all the profiles, use C(profile='').
            - Currently supported on Illumos/Solaris.
        type: str
        version_added: "2.8"
    authorization:
        description:
            - Sets the authorization of the user.
            - Does nothing when used with other platforms.
            - Can set multiple authorizations using comma separation.
            - To delete all authorizations, use C(authorization='').
            - Currently supported on Illumos/Solaris.
        type: str
        version_added: "2.8"
    role:
        description:
            - Sets the role of the user.
            - Does nothing when used with other platforms.
            - Can set multiple roles using comma separation.
            - To delete all roles, use C(role='').
            - Currently supported on Illumos/Solaris.
        type: str
        version_added: "2.8"
    password_expire_max: