### 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
lines.append(line)
                                continue
                            fields[1] = self.password
                            fields[2] = str(int(time.time() // 86400))
                            if minweeks:
                                try:
                                    fields[3] = str(int(minweeks) * 7)
                                except ValueError:
                                    
                                    pass
                            if maxweeks:
                                try:
                                    fields[4] = str(int(maxweeks) * 7)
                                except ValueError:
                                    
                                    pass
                            if warnweeks:
                                try:
                                    fields[5] = str(int(warnweeks) * 7)
                                except ValueError:
                                    
                                    pass
                            line = ':'.join(fields)
                            lines.append('%s\n' % line)
                    with open(self.SHADOWFILE, 'w+') as f:
                        f.writelines(lines)
                except Exception as err:
                    self.module.fail_json(msg="failed to update users password: %s" % to_native(err))
        return (rc, out, err)
    def modify_user_usermod(self):