### 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
"""
    This is the Alpine User manipulation class. It inherits the BusyBox class
    behaviors such as using adduser and deluser commands.
    """
    platform = 'Linux'
    distribution = 'Alpine'
def main():
    ssh_defaults = dict(
        bits=0,
        type='rsa',
        passphrase=None,
        comment='ansible-generated on %s' % socket.gethostname()
    )
    module = AnsibleModule(
        argument_spec=dict(
            state=dict(type='str', default='present', choices=['absent', 'present']),
            name=dict(type='str', required=True, aliases=['user']),
            uid=dict(type='int'),