### 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
test1:
                password = *
                lastupdate = 1553695126
        """
        b_name = to_bytes(self.name)
        b_passwd = b''
        b_expires = b''
        if os.path.exists(self.SHADOWFILE) and os.access(self.SHADOWFILE, os.R_OK):
            with open(self.SHADOWFILE, 'rb') as bf:
                b_lines = bf.readlines()
            b_passwd_line = b''
            b_expires_line = b''
            try:
                for index, b_line in enumerate(b_lines):
                    
                    if b_line.startswith(b'%s:' % b_name):
                        b_passwd_line = b_lines[index + 1]
                        b_expires_line = b_lines[index + 2]
                        break
                
                if b' = ' in b_passwd_line:
                    b_passwd = b_passwd_line.split(b' = ', 1)[-1].strip()
                if b' = ' in b_expires_line:
                    b_expires = b_expires_line.split(b' = ', 1)[-1].strip()
            except IndexError:
                self.module.fail_json(msg='Failed to parse shadow file %s' % self.SHADOWFILE)
        passwd = to_native(b_passwd)
        expires = to_native(b_expires) or -1
        return passwd, expires
class HPUX(User):