Snapshot Finalization Throws NPE in 7.5

[original-brownbear]

We're seeing the following NPE in Cloud logs:

Caused by: java.lang.NullPointerException
	at org.elasticsearch.repositories.blobstore.ChecksumBlobStoreFormat.write(ChecksumBlobStoreFormat.java:226) ~[elasticsearch-7.5.0.jar:7.5.0]
	at org.elasticsearch.repositories.blobstore.ChecksumBlobStoreFormat.writeTo(ChecksumBlobStoreFormat.java:197) ~[elasticsearch-7.5.0.jar:7.5.0]
	at org.elasticsearch.repositories.blobstore.ChecksumBlobStoreFormat.write(ChecksumBlobStoreFormat.java:185) ~[elasticsearch-7.5.0.jar:7.5.0]
	at org.elasticsearch.repositories.blobstore.BlobStoreRepository.lambda$finalizeSnapshot$26(BlobStoreRepository.java:738) ~[elasticsearch-7.5.0.jar:7.5.0]
	at org.elasticsearch.action.ActionRunnable$1.doRun(ActionRunnable.java:45) ~[elasticsearch-7.5.0.jar:7.5.0]
	... 5 more

during snapshot finalization. This seems like a bug where the cluster metadata does not contain an index for a name that was just snapshot (the deeper cause is that clusterMetaData.index(index.getName()) in BlobStoreRepository returns null). I'm investigating this.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)