### 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:
            - This only affects C(state=absent), it forces removal of the user and associated directories on supported platforms.
            - The behavior is the same as C(userdel --force), check the man page for C(userdel) on your system for details and support.
            - When used with C(generate_ssh_key=yes) this forces an existing key to be overwritten.
        type: bool
        default: no
    remove:
        description:
            - This only affects C(state=absent), it attempts to remove directories associated with the user.
            - The behavior is the same as C(userdel --remove), check the man page for details and support.
        type: bool
        default: no
    login_class:
        description:
            - Optionally sets the user's login class, a feature of most BSD OSs.
        type: str
    generate_ssh_key:
        description:
            - Whether to generate a SSH key for the user in question.
            - This will B(not) overwrite an existing SSH key unless used with C(force=yes).
        type: bool
        default: no
        version_added: "0.9"
    ssh_key_bits:
        description:
            - Optionally specify number of bits in SSH key to create.
            - The default value depends on ssh-keygen.
        type: int
        version_added: "0.9"
    ssh_key_type: