### 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 not self.user_exists():
            return False
        info = self.get_pwd_info()
        if len(info[1]) == 1 or len(info[1]) == 0:
            info[1] = self.user_password()[0]
        return info
    def set_password_expire(self):
        min_needs_change = self.password_expire_min is not None
        max_needs_change = self.password_expire_max is not None
        if HAVE_SPWD:
            try:
                shadow_info = getspnam(to_bytes(self.name))
            except ValueError:
                return None, '', ''
            min_needs_change &= self.password_expire_min != shadow_info.sp_min
            max_needs_change &= self.password_expire_max != shadow_info.sp_max
        if not (min_needs_change or max_needs_change):
            return (None, '', '')  
        command_name = 'chage'
        cmd = [self.module.get_bin_path(command_name, True)]
        if min_needs_change:
            cmd.extend(["-m", self.password_expire_min])
        if max_needs_change:
            cmd.extend(["-M", self.password_expire_max])
        cmd.append(self.name)
        return self.execute_command(cmd)
    def user_password(self):