### 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 not os.path.exists(path):
            if self.skeleton is not None:
                skeleton = self.skeleton
            else:
                skeleton = '/etc/skel'
            if os.path.exists(skeleton):
                try:
                    shutil.copytree(skeleton, path, symlinks=True)
                except OSError as e:
                    self.module.exit_json(failed=True, msg="%s" % to_native(e))
            else:
                try:
                    os.makedirs(path)
                except OSError as e:
                    self.module.exit_json(failed=True, msg="%s" % to_native(e))
            
            if os.path.exists(self.LOGIN_DEFS):
                with open(self.LOGIN_DEFS, 'r') as f:
                    for line in f:
                        m = re.match(r'^UMASK\s+(\d+)$', line)
                        if m:
                            umask = int(m.group(1), 8)
                            mode = 0o777 & ~umask
                            try:
                                os.chmod(path, mode)
                            except OSError as e:
                                self.module.exit_json(failed=True, msg="%s" % to_native(e))
    def chown_homedir(self, uid, gid, path):