### 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(self.password)
        (rc, out, err) = (None, '', '')
        
        if len(cmd) > 1:
            cmd.append(self.name)
            (rc, out, err) = self.execute_command(cmd)
        if not self.local or not (rc is None or rc == 0):
            return (rc, out, err)
        if lexpires is not None:
            (rc, _out, _err) = self.execute_command([lchage_cmd, '-E', to_native(lexpires), self.name])
            out += _out
            err += _err
            if rc != 0:
                return (rc, out, err)
        if len(lgroupmod_add) == 0 and len(lgroupmod_del) == 0:
            return (rc, out, err)
        for add_group in lgroupmod_add:
            (rc, _out, _err) = self.execute_command([lgroupmod_cmd, '-M', self.name, add_group])
            out += _out
            err += _err
            if rc != 0:
                return (rc, out, err)
        for del_group in lgroupmod_del:
            (rc, _out, _err) = self.execute_command([lgroupmod_cmd, '-m', self.name, del_group])
            out += _out
            err += _err
            if rc != 0:
                return (rc, out, err)
        return (rc, out, err)
    def group_exists(self, group):