allow connections with username only

Section [MQTT-3.1.2-22] of the MQTT 3.1.1 only says that a password cannot be
given without a user. The reverse should be allowed. Fixes #79.

amqtt/mqtt/protocol/broker_handler.py

@@ -163,8 +163,6 @@ class BrokerProtocolHandler(ProtocolHandler):
             )  # [MQTT-3.2.2-4] session_parent=0
         elif not connect.username_flag and connect.password_flag:
             connack = ConnackPacket.build(0, BAD_USERNAME_PASSWORD)  # [MQTT-3.1.2-22]
-        elif connect.username_flag and not connect.password_flag:
-            connack = ConnackPacket.build(0, BAD_USERNAME_PASSWORD)  # [MQTT-3.1.2-22]
         elif connect.username_flag and connect.username is None:
             error_msg = "Invalid username from %s" % (
                 format_client_message(address=remote_address, port=remote_port)

tests/test_client.py

@@ -111,6 +111,17 @@ async def test_connect_ws_secure():
     await broker.shutdown()
 
 
+@pytest.mark.asyncio
+async def test_connect_username_without_password():
+    broker = Broker(broker_config, plugin_namespace="amqtt.test.plugins")
+    await broker.start()
+    client = MQTTClient()
+    await client.connect("mqtt://alice@127.0.0.1/")
+    assert client.session is not None
+    await client.disconnect()
+    await broker.shutdown()
+
+
 @pytest.mark.asyncio
 async def test_ping():
     broker = Broker(broker_config, plugin_namespace="amqtt.test.plugins")