### 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
current_groups = self.user_group_membership()
            groups_need_mod = False
            groups_option = '-S'
            groups = []
            if self.groups == '':
                if current_groups and not self.append:
                    groups_need_mod = True
            else:
                groups = self.get_groups_set(names_only=True)
                group_diff = set(current_groups).symmetric_difference(groups)
                if group_diff:
                    if self.append:
                        for g in groups:
                            if g in group_diff:
                                groups_option = '-G'
                                groups_need_mod = True
                                break
                    else:
                        groups_need_mod = True
            if groups_need_mod:
                cmd.append(groups_option)
                cmd.append(','.join(groups))
        if self.comment is not None and info[4] != self.comment:
            cmd.append('-c')
            cmd.append(self.comment)
        if self.home is not None and info[5] != self.home:
            if self.move_home:
                cmd.append('-m')
            cmd.append('-d')
            cmd.append(self.home)