### 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
ssh_key_file = self.get_ssh_key_path()
        if not os.path.exists(ssh_key_file):
            return (1, 'SSH Key file %s does not exist' % ssh_key_file, '')
        cmd = [self.module.get_bin_path('ssh-keygen', True)]
        cmd.append('-l')
        cmd.append('-f')
        cmd.append(ssh_key_file)
        return self.execute_command(cmd, obey_checkmode=False)
    def get_ssh_public_key(self):
        ssh_public_key_file = '%s.pub' % self.get_ssh_key_path()
        try:
            with open(ssh_public_key_file, 'r') as f:
                ssh_public_key = f.read().strip()
        except IOError:
            return None
        return ssh_public_key
    def create_user(self):
        
        return self.create_user_useradd()
    def remove_user(self):
        
        return self.remove_user_userdel()
    def modify_user(self):
        
        return self.modify_user_usermod()
    def create_homedir(self, path):