### 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 = self._get_dscl()
        cmd += ['-search', '/Groups', 'GroupMembership', self.name]
        (rc, out, err) = self.execute_command(cmd, obey_checkmode=False)
        groups = []
        for line in out.splitlines():
            if line.startswith(' ') or line.startswith(')'):
                continue
            groups.append(line.split()[0])
        return groups
    def _get_user_property(self, property):
        '''Return user PROPERTY as given my dscl(1) read or None if not found.'''
        cmd = self._get_dscl()
        cmd += ['-read', '/Users/%s' % self.name, property]
        (rc, out, err) = self.execute_command(cmd, obey_checkmode=False)
        if rc != 0:
            return None
        
        
        
        lines = out.splitlines()
        
        if len(lines) == 1:
            return lines[0].split(': ')[1]
        if len(lines) > 2:
            return '\n'.join([lines[1].strip()] + lines[2:])
        if len(lines) == 2:
            return lines[1].strip()
        return None
    def _get_next_uid(self, system=None):