### 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
from __future__ import absolute_import, division, print_function
__metaclass__ = type
DOCUMENTATION = r'''
module: user
version_added: "0.2"
short_description: Manage user accounts
description:
    - Manage user accounts and user attributes.
    - For Windows targets, use the M(ansible.windows.win_user) module instead.
options:
    name:
        description:
            - Name of the user to create, remove or modify.
        type: str
        required: true
        aliases: [ user ]
    uid:
        description:
            - Optionally sets the I(UID) of the user.
        type: int
    comment:
        description:
            - Optionally sets the description (aka I(GECOS)) of user account.
        type: str
    hidden:
        description:
            - macOS only, optionally hide the user from the login window and system preferences.
            - The default will be C(true) if the I(system) option is used.