[Bug]: KafkaConnectUtils.debeziumRecordInstant() fails with NullPointerException on DELETE operations

[tstangenberg]

What happened?

The debeziumRecordInstant() method in KafkaConnectUtils throws a NullPointerException when processing Debezium DELETE events because it doesn't handle the case where valueSchema is null.

Steps to Reproduce

1. Set up a Debezium pipeline using DebeziumIO.read()
2. Perform a DELETE operation on a record in the source database
3. The pipeline crashes with a NullPointerException

Expected Behavior

The method should handle DELETE operations gracefully by extracting the timestamp from the sourceOffset metadata, specifically from the ts_usec field.

Actual Behavior

NullPointerException: Cannot invoke "org.apache.kafka.connect.data.Schema.type()" because the return value
        of "org.apache.kafka.connect.source.SourceRecord.valueSchema()" is null
	at org.apache.beam.io.debezium.KafkaConnectUtils.debeziumRecordInstant(KafkaConnectUtils.java:83)

Root Cause
For DELETE operations, Debezium sets and to null, but the timestamp information is available in sourceOffset.ts_usec: value``valueSchema

SourceRecord{
  sourcePartition={server=beam-debezium-connector}, 
  sourceOffset={lsn_proc=42600784, messageType=DELETE, lsn_commit=42600728, lsn=42600784, txId=1073, ts_usec=1772459804615258}
  ConnectRecord{..., value=null, valueSchema=null, ...}
}

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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