### 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
cmd.append('-c')
            cmd.append(self.comment)
        if self.home is not None:
            cmd.append('-d')
            cmd.append(self.home)
        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)
            cmd.append('-g')
            cmd.append(self.group)
        if self.groups is not None:
            groups = self.get_groups_set()
            cmd.append('-G')
            cmd.append(','.join(groups))
        if self.create_home:
            cmd.append('-m')
            if self.skeleton is not None:
                cmd.append('-k')
                cmd.append(self.skeleton)
            if self.umask is not None:
                cmd.append('-K')
                cmd.append('UMASK=' + self.umask)
        if self.shell is not None:
            cmd.append('-s')
            cmd.append(self.shell)
        if self.login_class is not None:
            cmd.append('-L')
            cmd.append(self.login_class)
        if self.expires is not None:
            cmd.append('-e')