### 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
if self.local:
            command_name = 'lusermod'
        else:
            command_name = 'usermod'
        usermod_path = self.module.get_bin_path(command_name, True)
        
        
        if not os.access(usermod_path, os.X_OK):
            return False
        cmd = [usermod_path, '--help']
        (rc, data1, data2) = self.execute_command(cmd, obey_checkmode=False)
        helpout = data1 + data2
        
        lines = to_native(helpout).split('\n')
        for line in lines:
            if line.strip().startswith('-a, --append'):
                return True
        return False
    def modify_user_usermod(self):