### 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
'''
        Return the next available uid. If system=True, then
        uid should be below of 500, if possible.
        '''
        cmd = self._get_dscl()
        cmd += ['-list', '/Users', 'UniqueID']
        (rc, out, err) = self.execute_command(cmd, obey_checkmode=False)
        if rc != 0:
            self.module.fail_json(
                msg="Unable to get the next available uid",
                rc=rc,
                out=out,
                err=err
            )
        max_uid = 0
        max_system_uid = 0
        for line in out.splitlines():
            current_uid = int(line.split(' ')[-1])
            if max_uid < current_uid:
                max_uid = current_uid
            if max_system_uid < current_uid and current_uid < 500:
                max_system_uid = current_uid
        if system and (0 < max_system_uid < 499):
            return max_system_uid + 1
        return max_uid + 1
    def _change_user_password(self):