### 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 generic User manipulation class that is subclassed
    based on platform.
    A subclass may wish to override the following action methods:-
      - create_user()
      - remove_user()
      - modify_user()
      - ssh_key_gen()
      - ssh_key_fingerprint()
      - user_exists()
    All subclasses MUST define platform and distribution (which may be None).
    """
    platform = 'Generic'
    distribution = None  
    PASSWORDFILE = '/etc/passwd'
    SHADOWFILE = '/etc/shadow'  
    SHADOWFILE_EXPIRE_INDEX = 7
    LOGIN_DEFS = '/etc/login.defs'
    DATE_FORMAT = '%Y-%m-%d'
    def __new__(cls, *args, **kwargs):