### 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
'''Convert SELF.GROUP to is stringed numerical value suitable for dscl.'''
        if self.group is None:
            self.group = 'nogroup'
        try:
            self.group = grp.getgrnam(self.group).gr_gid
        except KeyError:
            self.module.fail_json(msg='Group "%s" not found. Try to create it first using "group" module.' % self.group)
        
        self.group = str(self.group)
    def __modify_group(self, group, action):
        '''Add or remove SELF.NAME to or from GROUP depending on ACTION.
        ACTION can be 'add' or 'remove' otherwise 'remove' is assumed. '''
        if action == 'add':
            option = '-a'
        else:
            option = '-d'
        cmd = ['dseditgroup', '-o', 'edit', option, self.name, '-t', 'user', group]
        (rc, out, err) = self.execute_command(cmd)
        if rc != 0:
            self.module.fail_json(msg='Cannot %s user "%s" to group "%s".'
                                      % (action, self.name, group), err=err, out=out, rc=rc)
        return (rc, out, err)
    def _modify_group(self):