### 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
"""
    This is a NetBSD User manipulation class.
    Main differences are that NetBSD:-
     - has no concept of "system" account.
     - has no force delete user
    This overrides the following methods from the generic class:-
      - create_user()
      - remove_user()
      - modify_user()
    """
    platform = 'NetBSD'
    distribution = None
    SHADOWFILE = '/etc/master.passwd'
    def create_user(self):
        cmd = [self.module.get_bin_path('useradd', True)]
        if self.uid is not None:
            cmd.append('-u')
            cmd.append(self.uid)
            if self.non_unique:
                cmd.append('-o')
        if self.group is not None:
            if not self.group_exists(self.group):
                self.module.fail_json(msg="Group %s does not exist" % self.group)
            cmd.append('-g')
            cmd.append(self.group)
        if self.groups is not None:
            groups = self.get_groups_set()