### 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.module.params['password'] and self.platform != 'Darwin':
            maybe_invalid = False
            
            if self.module.params['password'] in set(['*', '!', '*************']):
                maybe_invalid = False
            else:
                
                
                if any(char in self.module.params['password'] for char in ':*!'):
                    maybe_invalid = True
                if '$' not in self.module.params['password']:
                    maybe_invalid = True
                else:
                    fields = self.module.params['password'].split("$")
                    if len(fields) >= 3:
                        
                        if bool(_HASH_RE.search(fields[-1])):
                            maybe_invalid = True
                        
                        if fields[1] == '1' and len(fields[-1]) != 22:
                            maybe_invalid = True
                        
                        if fields[1] == '5' and len(fields[-1]) != 43:
                            maybe_invalid = True
                        
                        if fields[1] == '6' and len(fields[-1]) != 86: