<!--- 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.
if not self.exists:
            return False
        query = "SELECT pg_drop_replication_slot('%s')" % self.name
        self.changed = exec_sql(self, query, ddl=True)
    def __slot_exists(self):
        query = "SELECT slot_type FROM pg_replication_slots WHERE slot_name = '%s'" % self.name
        res = exec_sql(self, query, add_to_executed=False)
        if res:
            self.exists = True
            self.kind = res[0][0]
#
def main():
    argument_spec = postgres_common_argument_spec()
    argument_spec.update(
        db=dict(type="str", aliases=["login_db"]),
        name=dict(type="str", aliases=["slot_name"]),
        slot_type=dict(type="str", default="physical", choices=["logical", "physical"]),
        immediately_reserve=dict(type="bool", default=False),
        session_role=dict(type="str"),
        output_plugin=dict(type="str", default="test_decoding"),
        state=dict(type="str", default="present", choices=["absent", "present"]),
    )
    module = AnsibleModule(
        argument_spec=argument_spec,
        supports_check_mode=True,
    )
    name = module.params["name"]
    slot_type = module.params["slot_type"]
    immediately_reserve = module.params["immediately_reserve"]