### 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
try:
            minweeks = ''
            maxweeks = ''
            warnweeks = ''
            with open("/etc/default/passwd", 'r') as f:
                for line in f:
                    line = line.strip()
                    if (line.startswith('#') or line == ''):
                        continue
                    m = re.match(r'^([^#]*)#(.*)$', line)
                    if m:  
                        line = m.group(1)
                    key, value = line.split('=')
                    if key == "MINWEEKS":
                        minweeks = value.rstrip('\n')
                    elif key == "MAXWEEKS":
                        maxweeks = value.rstrip('\n')
                    elif key == "WARNWEEKS":
                        warnweeks = value.rstrip('\n')
        except Exception as err:
            self.module.fail_json(msg="failed to read /etc/default/passwd: %s" % to_native(err))
        return (minweeks, maxweeks, warnweeks)
    def remove_user(self):
        cmd = [self.module.get_bin_path('userdel', True)]
        if self.remove:
            cmd.append('-r')
        cmd.append(self.name)
        return self.execute_command(cmd)
    def create_user(self):