### 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:
            - Maximum number of days between password change.
            - Supported on Linux only.
        type: int
        version_added: "2.11"
    password_expire_min:
        description:
            - Minimum number of days between password change.
            - Supported on Linux only.
        type: int
        version_added: "2.11"
    umask:
        description:
            - Sets the umask of the user.
            - Does nothing when used with other platforms.
            - Currently supported on Linux.
            - Requires C(local) is omitted or False.
        type: str
        version_added: "2.12"
extends_documentation_fragment: action_common_attributes
attributes:
    check_mode:
        support: full
    diff_mode:
        support: none
    platform:
        platforms: posix
notes:
  - There are specific requirements per platform on user management utilities. However
    they generally come pre-installed with the system and Ansible will require they