rlm_kafka: recognise a null key box on the xlat path

With the framework now carrying FR_TYPE_NULL boxes through to the
xlat body, check for fr_type_is_null() in addition to the zero-length
check so an explicit `null` key, an empty '' literal, and an
attribute that expanded to nothing all resolve to "no key on the wire"
without any of them needing to coerce into a zero-length octets value.

Author: arr2036

src/modules/rlm_kafka/rlm_kafka.c

@@ -805,12 +805,12 @@ static xlat_action_t kafka_xlat_produce(UNUSED TALLOC_CTX *xctx_ctx, UNUSED fr_d
 	}
 
 	/*
-	 *	Zero-length octets (e.g. `''` or an attribute expanding
-	 *	to nothing) map to "no key" on the wire - librdkafka then
-	 *	uses the configured partitioner instead of key-hash
-	 *	partitioning.
+	 *	Explicit `null`, a zero-length octets literal, or an
+	 *	attribute expanding to nothing all map to "no key" on
+	 *	the wire - librdkafka then uses the configured
+	 *	partitioner instead of key-hash partitioning.
 	 */
-	if (key_vb->vb_length > 0) {
+	if (!fr_type_is_null(key_vb->type) && key_vb->vb_length > 0) {
 		key = key_vb->vb_octets;
 		key_len = key_vb->vb_length;
 	}