### 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.ssh_bits)
        cmd.append('-C')
        cmd.append(self.ssh_comment)
        cmd.append('-f')
        cmd.append(ssh_key_file)
        if self.ssh_passphrase is not None:
            if self.module.check_mode:
                self.module.debug('In check mode, would have run: "%s"' % cmd)
                return (0, '', '')
            master_in_fd, slave_in_fd = pty.openpty()
            master_out_fd, slave_out_fd = pty.openpty()
            master_err_fd, slave_err_fd = pty.openpty()
            env = os.environ.copy()
            env['LC_ALL'] = get_best_parsable_locale(self.module)
            try:
                p = subprocess.Popen([to_bytes(c) for c in cmd],
                                     stdin=slave_in_fd,
                                     stdout=slave_out_fd,
                                     stderr=slave_err_fd,
                                     preexec_fn=os.setsid,
                                     env=env)
                out_buffer = b''
                err_buffer = b''
                while p.poll() is None:
                    r_list = select.select([master_out_fd, master_err_fd], [], [], 1)[0]
                    first_prompt = b'Enter passphrase (empty for no passphrase):'
                    second_prompt = b'Enter same passphrase again'
                    prompt = first_prompt
                    for fd in r_list:
                        if fd == master_out_fd: