### 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'
            lgroupmod_cmd = self.module.get_bin_path('lgroupmod', True)
            lgroupmod_add = set()
            lgroupmod_del = set()
            lchage_cmd = self.module.get_bin_path('lchage', True)
            lexpires = None
        else:
            command_name = 'usermod'
        cmd = [self.module.get_bin_path(command_name, True)]
        info = self.user_info()
        has_append = self._check_usermod_append()
        if self.uid is not None and info[2] != int(self.uid):
            cmd.append('-u')
            cmd.append(self.uid)