[BUG] S3 Scan hits NPE when bucket key has a null value

[graytaylor0]

Describe the bug
It is possible for a NPE to occur when the global state Map for s3 scan contains a key for the bucket but the value is null (

data-prepper/data-prepper-plugins/s3-source/src/main/java/org/opensearch/dataprepper/plugins/source/S3ScanPartitionCreationSupplier.java

Line 104 in cd194c1

Instant mostRecentLastModifiedTimestamp = globalStateMap.containsKey(bucket) ? Instant.parse((String) globalStateMap.get(bucket)) : null;

)

2023-09-08T15:42:56.313 [Thread-10] ERROR org.opensearch.dataprepper.plugins.source.ScanObjectWorker - Received an exception while processing S3 objects, backing off and retrying
java.lang.NullPointerException: text
	at java.util.Objects.requireNonNull(Objects.java:246) ~[?:?]
	at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1945) ~[?:?]
	at java.time.Instant.parse(Instant.java:395) ~[?:?]
	at org.opensearch.dataprepper.plugins.source.S3ScanPartitionCreationSupplier.listFilteredS3ObjectsForBucket(S3ScanPartitionCreationSupplier.java:104) ~[s3-source-2.4.0.jar:?]
	at org.opensearch.dataprepper.plugins.source.S3ScanPartitionCreationSupplier.apply(S3ScanPartitionCreationSupplier.java:87) ~[s3-source-2.4.0.jar:?]
	at org.opensearch.dataprepper.plugins.source.S3ScanPartitionCreationSupplier.apply(S3ScanPartitionCreationSupplier.java:32) ~[s3-source-2.4.0.jar:?]
	at org.opensearch.dataprepper.sourcecoordination.LeaseBasedSourceCoordinator.getNextPartition(LeaseBasedSourceCoordinator.java:153) ~[data-prepper-core-2.4.0.jar:?]
	at org.opensearch.dataprepper.plugins.source.ScanObjectWorker.startProcessingObject(ScanObjectWorker.java:128) ~[s3-source-2.4.0.jar:?]
	at org.opensearch.dataprepper.plugins.source.ScanObjectWorker.run(ScanObjectWorker.java:106) ~[s3-source-2.4.0.jar:?]
	at java.lang.Thread.run(Thread.java:829) [?:?]

Expected behavior
No NullPointerException

** Steps to reproduce **
Configure an s3 scan pipeline with the same bucket duplicated twice

- bucket:
    name: "my-bucket"
- bucket:
    name: "my-bucket"

Screenshots
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Environment (please complete the following information):

• OS: [e.g. Ubuntu 20.04 LTS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.