Fix no-payload commit in consumer

The confluent-kafka `Consumer.commit` method can be called with no
parameters, then the current partition assignment's offsets are used
(see the documentation at
https://docs.confluent.io/platform/current/clients/confluent-kafka-python/html/index.html#confluent_kafka.Consumer.commit).

The previous behaviour was erroneous, raising a `TypeError`, thus an
HTTP 500 when in the REST proxy's consumer manager a commit is performed
with an empty payload.

karapace/kafka/consumer.py

@@ -86,7 +86,10 @@ def commit(  # type: ignore[override]
             if message is not None:
                 return super().commit(message=message, asynchronous=False)
 
-            return super().commit(offsets=offsets, asynchronous=False)
+            if offsets is not None:
+                return super().commit(offsets=offsets, asynchronous=False)
+
+            return super().commit(asynchronous=False)
         except KafkaException as exc:
             raise_from_kafkaexception(exc)