fix: Fix NPE when Kafka Connect requests a commit right after an error (#23)

Author: jerrinot

connector/src/main/java/io/questdb/kafka/QuestDBSinkTask.java

@@ -467,6 +467,18 @@ private boolean tryWriteLogicalType(String name, Schema schema, Object value) {
         return false;
     }
 
+    @Override
+    public Map<TopicPartition, OffsetAndMetadata> preCommit(Map<TopicPartition, OffsetAndMetadata> currentOffsets) {
+        if (sender != null) {
+            flush(currentOffsets);
+            return currentOffsets;
+        } else {
+            // null sender indicates there was an error and we cannot guarantee that the data was actually sent
+            // returning empty map will cause the task to avoid committing offsets to Kafka
+            return Collections.emptyMap();
+        }
+    }
+
     @Override
     public void flush(Map<TopicPartition, OffsetAndMetadata> map) {
         if (httpTransport) {