### 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
'''Add or remove SELF.NAME to or from GROUP depending on ACTION.
        ACTION can be 'add' or 'remove' otherwise 'remove' is assumed. '''
        rc = 0
        out = ''
        err = ''
        changed = False
        current = set(self._list_user_groups())
        if self.groups is not None:
            target = set(self.groups.split(','))
        else:
            target = set([])
        if self.append is False:
            for remove in current - target:
                (_rc, _out, _err) = self.__modify_group(remove, 'delete')
                rc += rc
                out += _out
                err += _err
                changed = True
        for add in target - current:
            (_rc, _out, _err) = self.__modify_group(add, 'add')
            rc += _rc
            out += _out
            err += _err
            changed = True
        return (rc, out, err, changed)
    def _update_system_user(self):