### 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
try:
            os.chown(path, uid, gid)
            for root, dirs, files in os.walk(path):
                for d in dirs:
                    os.chown(os.path.join(root, d), uid, gid)
                for f in files:
                    os.chown(os.path.join(root, f), uid, gid)
        except OSError as e:
            self.module.exit_json(failed=True, msg="%s" % to_native(e))
class FreeBsdUser(User):
    """
    This is a FreeBSD User manipulation class - it uses the pw command
    to manipulate the user database, followed by the chpass command
    to change the password.
    This overrides the following methods from the generic class:-
      - create_user()
      - remove_user()
      - modify_user()
    """
    platform = 'FreeBSD'
    distribution = None
    SHADOWFILE = '/etc/master.passwd'
    SHADOWFILE_EXPIRE_INDEX = 6
    DATE_FORMAT = '%d-%b-%Y'
    def _handle_lock(self):