NPE when header value is null

Hi, I'm exploring the option to have a backup to cold storage (azure blob) using the blob sink connector. In general, working great.

However, I'm facing a problem in case of records with headers where the value is null:

`org.apache.kafka.connect.errors.ConnectException: Failed to write records to Azure Blob
at io.aiven.kafka.connect.azure.sink.AzureBlobSinkTask.flushFile(AzureBlobSinkTask.java:163)
at java.base/java.util.HashMap.forEach(HashMap.java:1337)
at java.base/java.util.Collections$UnmodifiableMap.forEach(Collections.java:1505)
at io.aiven.kafka.connect.azure.sink.AzureBlobSinkTask.flush(AzureBlobSinkTask.java:130)
at org.apache.kafka.connect.sink.SinkTask.preCommit(SinkTask.java:125)
at org.apache.kafka.connect.runtime.WorkerSinkTask.commitOffsets(WorkerSinkTask.java:404)
at org.apache.kafka.connect.runtime.WorkerSinkTask.commitOffsets(WorkerSinkTask.java:374)
at org.apache.kafka.connect.runtime.WorkerSinkTask.iteration(WorkerSinkTask.java:218)
at org.apache.kafka.connect.runtime.WorkerSinkTask.execute(WorkerSinkTask.java:203)
at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:188)
at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:243)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.NullPointerException
at java.base/java.util.Base64$Encoder.encode(Base64.java:267)
at io.aiven.kafka.connect.common.output.plainwriter.HeadersPlainWriter.write(HeadersPlainWriter.java:48)
at io.aiven.kafka.connect.common.output.plainwriter.PlainOutputStreamWriter.writeOneRecord(PlainOutputStreamWriter.java:51)
at io.aiven.kafka.connect.common.output.OutputWriter.writeRecord(OutputWriter.java:88)
at io.aiven.kafka.connect.common.output.OutputWriter.writeRecords(OutputWriter.java:76)
at io.aiven.kafka.connect.azure.sink.AzureBlobSinkTask.flushFile(AzureBlobSinkTask.java:156)
... 15 more`

This can be reproduced by sinking following record (with a correlation_id header set to null):

`{
"Value": "{ ... }",
"Offset": 343,
"Key": "{ ... }",
"Partition": 0,
"Headers": {
"correlation_id": null,
"Timestamp": "2025-04-10T08:56:44.26Z"
}`

The HeadersPlainWriter is assuming all header values are non-null. Should we make this null-safe?