### 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
maybe_invalid = True
                    else:
                        maybe_invalid = True
            if maybe_invalid:
                self.module.warn("The input password appears not to have been hashed. "
                                 "The 'password' argument must be encrypted for this module to work properly.")
    def execute_command(self, cmd, use_unsafe_shell=False, data=None, obey_checkmode=True):
        if self.module.check_mode and obey_checkmode:
            self.module.debug('In check mode, would have run: "%s"' % cmd)
            return (0, '', '')
        else:
            
            cmd = [str(x) for x in cmd]
            return self.module.run_command(cmd, use_unsafe_shell=use_unsafe_shell, data=data)
    def backup_shadow(self):
        if not self.module.check_mode and self.SHADOWFILE:
            return self.module.backup_local(self.SHADOWFILE)
    def remove_user_userdel(self):
        if self.local:
            command_name = 'luserdel'
        else:
            command_name = 'userdel'
        cmd = [self.module.get_bin_path(command_name, True)]
        if self.force and not self.local:
            cmd.append('-f')
        if self.remove:
            cmd.append('-r')
        cmd.append(self.name)
        return self.execute_command(cmd)
    def create_user_useradd(self):