### 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.append('-k')
                cmd.append(self.skeleton)
            if self.umask is not None:
                cmd.append('-K')
                cmd.append('UMASK=' + self.umask)
        if self.profile is not None:
            cmd.append('-P')
            cmd.append(self.profile)
        if self.authorization is not None:
            cmd.append('-A')
            cmd.append(self.authorization)
        if self.role is not None:
            cmd.append('-R')
            cmd.append(self.role)
        cmd.append(self.name)
        (rc, out, err) = self.execute_command(cmd)
        if rc is not None and rc != 0:
            self.module.fail_json(name=self.name, msg=err, rc=rc)
        if not self.module.check_mode:
            
            if self.password is not None:
                self.backup_shadow()
                minweeks, maxweeks, warnweeks = self.get_password_defaults()
                try:
                    lines = []
                    with open(self.SHADOWFILE, 'rb') as f:
                        for line in f:
                            line = to_native(line, errors='surrogate_or_strict')
                            fields = line.strip().split(':')
                            if not fields[0] == self.name: