### 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: bool
        version_added: "2.6"
    non_unique:
        description:
            - Optionally when used with the -u option, this option allows to change the user ID to a non-unique value.
        type: bool
        default: no
        version_added: "1.1"
    seuser:
        description:
            - Optionally sets the seuser type (user_u) on selinux enabled systems.
        type: str
        version_added: "2.1"
    group:
        description:
            - Optionally sets the user's primary group (takes a group name).
        type: str
    groups:
        description:
            - List of groups user will be added to.
            - By default, the user is removed from all other groups. Configure C(append) to modify this.
            - When set to an empty string C(''),
              the user is removed from all groups except the primary group.
            - Before Ansible 2.3, the only input format allowed was a comma separated string.
        type: list
        elements: str
    append:
        description:
            - If C(true), add the user to the groups specified in C(groups).
            - If C(false), user will only be added to the groups specified in C(groups),