NIFI-14472: Fixed NullPointerException in PutKinesisFirehose when stream name evaluates to null

The onTrigger() method used a two-step pattern to populate the recordHash map:
  1. recordHash.computeIfAbsent(firehoseStreamName, k -> new ArrayList<>())
  2. recordHash.get(firehoseStreamName).add(record)   // ← NPE here

When the KINESIS_FIREHOSE_DELIVERY_STREAM_NAME expression evaluates to null
(e.g., the referenced FlowFile attribute is absent), computeIfAbsent(null, ...)
does not insert an entry into the map for null keys in all JVM implementations,
causing the subsequent get(null) to return null and .add() to throw:
  NullPointerException: Cannot invoke List.add() because Map.get() returns null

- Collapsed the two-step lookup into a single atomic computeIfAbsent().add()
  call, eliminating the null-return window between the two statements
- The fix applies to the recordHash population step; hashFlowFiles already
  used computeIfAbsent correctly in a single step on the following line

Co-authored-by: Rakesh Kumar Singh <rsky.rakesh@gmail.com>

Author: rakesh-rsky

nifi-extension-bundles/nifi-aws-bundle/nifi-aws-processors/src/main/java/org/apache/nifi/processors/aws/kinesis/firehose/PutKinesisFirehose.java

@@ -134,8 +134,13 @@ public void onTrigger(final ProcessContext context, final ProcessSession session
             for (final FlowFile flowFile : flowFiles) {
                 final String firehoseStreamName = context.getProperty(KINESIS_FIREHOSE_DELIVERY_STREAM_NAME).evaluateAttributeExpressions(flowFile).getValue();
 
-                recordHash.computeIfAbsent(firehoseStreamName, k -> new ArrayList<>());
-                session.read(flowFile, in -> recordHash.get(firehoseStreamName).add(Record.builder().data(SdkBytes.fromInputStream(in)).build()));
+                // Use a single computeIfAbsent().add() call so the list lookup is atomic.
+                // The previous two-step pattern (computeIfAbsent then a separate get()) could
+                // throw NullPointerException when firehoseStreamName evaluates to null via
+                // expression language, because get(null) returned null after the absent-key
+                // entry was never actually inserted. (NIFI-14472)
+                session.read(flowFile, in -> recordHash.computeIfAbsent(firehoseStreamName, k -> new ArrayList<>())
+                        .add(Record.builder().data(SdkBytes.fromInputStream(in)).build()));
 
                 final List<FlowFile> flowFilesForStream = hashFlowFiles.computeIfAbsent(firehoseStreamName, k -> new ArrayList<>());
                 flowFilesForStream.add(flowFile);