### 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
group_diff = set(current_groups).symmetric_difference(groups)
            groups_need_mod = False
            if group_diff:
                if self.append:
                    for g in groups:
                        if g in group_diff:
                            groups_need_mod = True
                            break
                else:
                    groups_need_mod = True
            if groups_need_mod:
                cmd.append('-G')
                new_groups = groups
                if self.append:
                    new_groups = groups | set(current_groups)
                cmd.append(','.join(new_groups))
        if self.expires is not None:
            current_expires = int(self.user_password()[1])
            
            
            if self.expires <= time.gmtime(0):
                if current_expires > 0:
                    cmd.append('-e')
                    cmd.append('0')
            else:
                
                current_expire_date = time.gmtime(current_expires)
                
                if current_expires <= 0 or current_expire_date[:3] != self.expires[:3]:
                    cmd.append('-e')