### 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 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.update(current_groups)
                cmd.append(','.join(new_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)
        if self.shell is not None and info[6] != self.shell:
            cmd.append('-s')
            cmd.append(self.shell)
        if self.profile is not None and info[7] != self.profile:
            cmd.append('-P')
            cmd.append(self.profile)
        if self.authorization is not None and info[8] != self.authorization:
            cmd.append('-A')