### 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.local:
            if not os.path.exists(self.PASSWORDFILE):
                self.module.fail_json(msg="'local: true' specified but unable to find local account file {0} to parse.".format(self.PASSWORDFILE))
            exists = False
            name_test = '{0}:'.format(self.name)
            with open(self.PASSWORDFILE, 'rb') as f:
                reversed_lines = f.readlines()[::-1]
                for line in reversed_lines:
                    if line.startswith(to_bytes(name_test)):
                        exists = True
                        break
            if not exists:
                self.module.warn(
                    "'local: true' specified and user '{name}' was not found in {file}. "
                    "The local user account may already exist if the local account database exists "
                    "somewhere other than {file}.".format(file=self.PASSWORDFILE, name=self.name))
            return exists
        else:
            try:
                if pwd.getpwnam(self.name):
                    return True
            except KeyError:
                return False
    def get_pwd_info(self):