Make init_kafka_producer handle empty broker URL

This is for the case of letting app initialization run even if a
producer hasn't been configured. In this scenario, the app would need to
explicitly test that safkir/kafka_producer is not None whenever it is
being tested.

src/safir/events.py

@@ -113,6 +113,9 @@ async def init_kafka_producer(app: Application) -> AsyncGenerator:
     This initializer adds an `aiokafka.AIOKafkaProducer` instance to the
     ``app`` under the ``safir/kafka_producer`` key.
 
+    If the ``kafka_broker_url`` configuration key has a value of `None`, then
+    the value of ``safir/kafka_producer`` is `None`.
+
     Examples
     --------
     Use this function as a `cleanup context
@@ -124,21 +127,30 @@ async def init_kafka_producer(app: Application) -> AsyncGenerator:
 
        producer = app["safir/kafka_producer"]
     """
-    # Startup phase
     logger = structlog.get_logger(app["safir/config"].logger_name)
-    logger.info("Starting Kafka producer")
-    producer = AIOKafkaProducer(
-        loop=asyncio.get_running_loop(),
-        bootstrap_servers=app["safir/config"].kafka_broker_url,
-        ssl_context=app["safir/kafka_ssl_context"],
-        security_protocol=app["safir/kafka_protocol"],
-    )
-    await producer.start()
-    app["safir/kafka_producer"] = producer
-    logger.info("Finished starting Kafka producer")
+
+    # Startup phase
+    kafka_broker_url = app["safir/config"].kafka_broker_url
+
+    if kafka_broker_url is None:
+        logger.info("Skipping Kafka producer initialization")
+        producer = None
+
+    else:
+        logger.info("Starting Kafka producer")
+        producer = AIOKafkaProducer(
+            loop=asyncio.get_running_loop(),
+            bootstrap_servers=kafka_broker_url,
+            ssl_context=app["safir/kafka_ssl_context"],
+            security_protocol=app["safir/kafka_protocol"],
+        )
+        await producer.start()
+        app["safir/kafka_producer"] = producer
+        logger.info("Finished starting Kafka producer")
 
     yield
 
     # Cleanup phase
-    logger.info("Shutting down Kafka producer")
-    await producer.stop()
+    if producer is not None:
+        logger.info("Shutting down Kafka producer")
+        await producer.stop()