<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### SUMMARY
<!--- Explain the problem briefly below -->

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->

postgresql_slot, postgresql_user

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.3
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['~/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python3.7/site-packages/ansible
  executable location = /usr/bin/ansible
  python version = 3.7.4 (default, Jul 16 2019, 07:12:58) [GCC 9.1.0]
```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
archlinux latest

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml
- postgresql_user:
    name: user1
    password: password
  become: true
  become_user: postgres

- postgresql_slot:
    name: slot1
  become: true
  become_user: postgres
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
Consistent warnings. Either both postgresql_user and postgresql_slot should give a warning or neither should.

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->
postgresql_slot gives warning "[WARNING]: Database name has not been passed, used default database to connect to." but postgresql_user doesn't, even though both of them have not specified a database.
for group in self.groups:
            self.revoked[group] = []
            for role in self.target_roles:
                
                if not self.__check_membership(group, role):
                    continue
                query = "REVOKE %s FROM %s" % ((pg_quote_identifier(group, 'role'),
                                               (pg_quote_identifier(role, 'role'))))
                self.changed = exec_sql(self, query, ddl=True)
                if self.changed:
                    self.revoked[group].append(role)
        return self.changed
    def __check_membership(self, src_role, dst_role):
        query = ("SELECT ARRAY(SELECT b.rolname FROM "
                 "pg_catalog.pg_auth_members m "
                 "JOIN pg_catalog.pg_roles b ON (m.roleid = b.oid) "
                 "WHERE m.member = r.oid) "
                 "FROM pg_catalog.pg_roles r "
                 "WHERE r.rolname = '%s'" % dst_role)
        res = exec_sql(self, query, add_to_executed=False)
        membership = []
        if res:
            membership = res[0][0]
        if not membership:
            return False
        if src_role in membership:
            return True
        return False
    def __check_roles_exist(self):