### 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.non_unique:
                cmd.append('-o')
        if self.group is not None:
            if not self.group_exists(self.group):
                self.module.fail_json(msg="Group %s does not exist" % self.group)
            ginfo = self.group_info(self.group)
            if info[3] != ginfo[2]:
                cmd.append('-g')
                cmd.append(ginfo[2])
        if self.groups is not None:
            
            current_groups = self.user_group_membership(exclude_primary=False)
            groups_need_mod = False
            groups = []
            if self.groups == '':
                if current_groups and not self.append:
                    groups_need_mod = True
            else:
                groups = self.get_groups_set(remove_existing=False, 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:
                                if has_append:
                                    cmd.append('-a')
                                groups_need_mod = True
                                break
                    else:
                        groups_need_mod = True