### 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
removing them from all other groups.
        type: bool
        default: no
    shell:
        description:
            - Optionally set the user's shell.
            - On macOS, before Ansible 2.5, the default shell for non-system users was C(/usr/bin/false).
              Since Ansible 2.5, the default shell for non-system users on macOS is C(/bin/bash).
            - See notes for details on how other operating systems determine the default shell by
              the underlying tool.
        type: str
    home:
        description:
            - Optionally set the user's home directory.
        type: path
    skeleton:
        description:
            - Optionally set a home skeleton directory.
            - Requires C(create_home) option!
        type: str
        version_added: "2.0"
    password:
        description:
            - If provided, set the user's password to the provided encrypted hash (Linux) or plain text password (macOS).
            - B(Linux/Unix/POSIX:) Enter the hashed password as the value.
            - See L(FAQ entry,https://docs.ansible.com/ansible/latest/reference_appendices/faq.html#how-do-i-generate-encrypted-passwords-for-the-user-module)
              for details on various ways to generate the hash of a password.
            - To create an account with a locked/disabled password on Linux systems, set this to C('!') or C('*').
            - To create an account with a locked/disabled password on OpenBSD, set this to C('*************').
            - B(OS X/macOS:) Enter the cleartext password as the value. Be sure to take relevant security precautions.