### 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(self.home)
        if self.shell is not None and info[6] != self.shell:
            cmd.append('-s')
            cmd.append(self.shell)
        
        if len(cmd) == 1:
            (rc, out, err) = (None, '', '')
        else:
            cmd.append(self.name)
            (rc, out, err) = self.execute_command(cmd)
        
        if self.update_password == 'always' and self.password is not None and info[1] != self.password:
            cmd = []
            cmd.append(self.module.get_bin_path('chpasswd', True))
            cmd.append('-e')
            cmd.append('-c')
            (rc2, out2, err2) = self.execute_command(cmd, data="%s:%s" % (self.name, self.password))
        else:
            (rc2, out2, err2) = (None, '', '')
        if rc is not None:
            return (rc, out + out2, err + err2)
        else:
            return (rc2, out + out2, err + err2)
    def parse_shadow_file(self):
        """Example AIX shadowfile data:
        nobody:
                password = *
        operator1:
                password = {ssha512}06$xxxxxxxxxxxx....
                lastupdate = 1549558094