### 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
passwd = ''
        expires = ''
        if HAVE_SPWD:
            try:
                shadow_info = getspnam(to_bytes(self.name))
                passwd = to_native(shadow_info.sp_pwdp)
                expires = shadow_info.sp_expire
                return passwd, expires
            except ValueError:
                return passwd, expires
        if not self.user_exists():
            return passwd, expires
        elif self.SHADOWFILE:
            passwd, expires = self.parse_shadow_file()
        return passwd, expires
    def parse_shadow_file(self):