### 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
sample: 'chrony,apache'
home:
  description: "Path to user's home directory."
  returned: When I(state) is C(present)
  type: str
  sample: '/home/asmith'
move_home:
  description: Whether or not to move an existing home directory.
  returned: When I(state) is C(present) and user exists
  type: bool
  sample: False
name:
  description: User account name.
  returned: always
  type: str
  sample: asmith
password:
  description: Masked value of the password.
  returned: When I(state) is C(present) and I(password) is not empty
  type: str
  sample: 'NOT_LOGGING_PASSWORD'
remove:
  description: Whether or not to remove the user account.
  returned: When I(state) is C(absent) and user exists
  type: bool
  sample: True
shell:
  description: User login shell.
  returned: When I(state) is C(present)
  type: str