Fix NPE in Partial Snapshot Without Global State (#55776) (#55783)

We make sure to filter shard generations for indices that are missing
from the metadata when finalizing a partial snapshot (from concurrent index deletion)
but we failed to account for the case where we manually build a fake metadata instance
for snapshots without the global state.
Fixed this by handling missing indices by skipping, same way we do it for filtering the
shard generations.

Relates #50234

Author: original-brownbear

server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java

@@ -494,7 +494,12 @@ private static Metadata metadataForSnapshot(SnapshotsInProgress.Entry snapshot,
             // Remove global state from the cluster state
             Metadata.Builder builder = Metadata.builder();
             for (IndexId index : snapshot.indices()) {
-                builder.put(metadata.index(index.getName()), false);
+                final IndexMetadata indexMetadata = metadata.index(index.getName());
+                if (indexMetadata == null) {
+                    assert snapshot.partial() : "Index [" + index + "] was deleted during a snapshot but snapshot was not partial.";
+                } else {
+                    builder.put(indexMetadata, false);
+                }
             }
             metadata = builder.build();
         }

server/src/test/java/org/elasticsearch/snapshots/SnapshotResiliencyTests.java

@@ -693,7 +693,7 @@ public void testConcurrentSnapshotDeleteAndDeleteIndex() throws IOException {
 
         continueOrDie(createIndicesListener, createIndexResponses ->
             client().admin().cluster().prepareCreateSnapshot(repoName, snapshotName).setWaitForCompletion(false)
-                .setPartial(partialSnapshot).execute(createSnapshotResponseStepListener));
+                .setPartial(partialSnapshot).setIncludeGlobalState(randomBoolean()).execute(createSnapshotResponseStepListener));
 
         continueOrDie(createSnapshotResponseStepListener,
             createSnapshotResponse -> client().admin().indices().delete(new DeleteIndexRequest(index),