Adding null check to handle rare NPE case

[preston-m-price]

Pull Request Description

In our deployment we rarely hit an NPE here, which typically results in duplicates due to the unhandled exception causing a premature exit from the WorkerSinkTask.commitOffsets method. In this failure mode we've already published batches to kusto, but we don't update the offsets due to an exception here.

I have not yet fully understood how we get into this state, where a TopicPartitionWriter doesn't exist for a TopicPartition.

Future Release Comment

Fixed the source of a NullPointerException that could result in duplicate records sent to Kusto.

Breaking Changes:

• None

Features:

• None

Fixes:

• None