### 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
if self.groups is None:
            return None
        info = self.user_info()
        groups = set(x.strip() for x in self.groups.split(',') if x)
        group_names = set()
        for g in groups.copy():
            if not self.group_exists(g):
                self.module.fail_json(msg="Group %s does not exist" % (g))
            group_info = self.group_info(g)
            if info and remove_existing and group_info[2] == info[3]:
                groups.remove(g)
            elif names_only:
                group_names.add(group_info[0])
        if names_only:
            return group_names
        return groups
    def user_group_membership(self, exclude_primary=True):
        ''' Return a list of groups the user belongs to '''
        groups = []
        info = self.get_pwd_info()
        for group in grp.getgrall():
            if self.name in group.gr_mem:
                
                if not exclude_primary:
                    groups.append(group[0])
                else:
                    if info[3] != group.gr_gid:
                        groups.append(group[0])
        return groups
    def user_exists(self):