### 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: '/bin/bash'
ssh_fingerprint:
  description: Fingerprint of generated SSH key.
  returned: When I(generate_ssh_key) is C(True)
  type: str
  sample: '2048 SHA256:aYNHYcyVm87Igh0IMEDMbvW0QDlRQfE0aJugp684ko8 ansible-generated on host (RSA)'
ssh_key_file:
  description: Path to generated SSH private key file.
  returned: When I(generate_ssh_key) is C(True)
  type: str
  sample: /home/asmith/.ssh/id_rsa
ssh_public_key:
  description: Generated SSH public key file.
  returned: When I(generate_ssh_key) is C(True)
  type: str
  sample: >
    'ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC95opt4SPEC06tOYsJQJIuN23BbLMGmYo8ysVZQc4h2DZE9ugbjWWGS1/pweUGjVstgzMkBEeBCByaEf/RJKNecKRPeGd2Bw9DCj/bn5Z6rGfNENKBmo
    618mUJBvdlEgea96QGjOwSB7/gmonduC7gsWDMNcOdSE3wJMTim4lddiBx4RgC9yXsJ6Tkz9BHD73MXPpT5ETnse+A3fw3IGVSjaueVnlUyUmOBf7fzmZbhlFVXf2Zi2rFTXqvbdGHKkzpw1U8eB8xFPP7y
    d5u1u0e6Acju/8aZ/l17IDFiLke5IzlqIMRTEbDwLNeO84YQKWTm9fODHzhYe0yvxqLiK07 ansible-generated on host'
stderr:
  description: Standard error from running commands.
  returned: When stderr is returned by a command that is run
  type: str
  sample: Group wheels does not exist
stdout:
  description: Standard output from running commands.
  returned: When standard output is returned by the command that is run
  type: str
  sample:
system: