### Summary

When using the user module to modify password_expire_min=0, the processing result does not meet expectations .
I guess the problem is caused by the following code in the user module.
```
    # deal with password expire min
    if user.password_expire_min:
        if user.user_exists():
            (rc, out, err) = user.set_password_expire_min()
```
Modifying to the following  code may fix this problem.
```
    # deal with password expire min
    if user.password_expire_min is not None:
        if user.user_exists():
            (rc, out, err) = user.set_password_expire_min()
```

### Issue Type

Bug Report

### Component Name

user

### Ansible Version

```console
$ ansible --version
ansible [core 2.11.1] 
  config file = None
  configured module search path = ['/home/betadmin/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /app/python3/lib/python3.8/site-packages/ansible
  ansible collection location = /home/betadmin/.ansible/collections:/usr/share/ansible/collections
  executable location = /app/python3/bin/ansible
  python version = 3.8.5 (default, Sep 18 2020, 14:55:53) [GCC 4.8.5 20150623 (Red Hat 4.8.5-16)]
  jinja version = 2.11.2
  libyaml = False
```


### Configuration

```console
$ ansible-config dump --only-changed
```


### OS / Environment

RHEL 7.4

### Steps to Reproduce

- name: Edit password_expire_min
  user:
    name: test
    password_expire_min: 0



### Expected Results

Minimum number of days between password change	: 0


### Actual Results


Minimum number of days between password change	: 2


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
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:
  description: Whether or not the account is a system account.
  returned: When I(system) is passed to the module and the account does not exist
  type: bool
  sample: True
uid:
  description: User ID of the user account.
  returned: When I(uid) is passed to the module
  type: int
  sample: 1044
password_expire_max:
  description: Maximum number of days during which a password is valid.
  returned: When user exists
  type: int
  sample: 20
password_expire_min: