### 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
RETURN = r'''
append:
  description: Whether or not to append the user to groups.
  returned: When state is C(present) and the user exists
  type: bool
  sample: True
comment:
  description: Comment section from passwd file, usually the user name.
  returned: When user exists
  type: str
  sample: Agent Smith
create_home:
  description: Whether or not to create the home directory.
  returned: When user does not exist and not check mode
  type: bool
  sample: True
force:
  description: Whether or not a user account was forcibly deleted.
  returned: When I(state) is C(absent) and user exists
  type: bool
  sample: False
group:
  description: Primary user group ID
  returned: When user exists
  type: int
  sample: 1001
groups:
  description: List of groups of which the user is a member.
  returned: When I(groups) is not empty and I(state) is C(present)
  type: str