Fix NPE in S3BlobContainer (#96168)

Relates #96019 Closes #96162

Author: DaveCTurner

modules/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3BlobContainer.java

@@ -803,7 +803,7 @@ public void compareAndExchangeRegister(
         ActionListener.run(ActionListener.releaseAfter(listener.delegateResponse((delegate, e) -> {
             if (e instanceof AmazonS3Exception amazonS3Exception && amazonS3Exception.getStatusCode() == 404) {
                 // an uncaught 404 means that our multipart upload was aborted by a concurrent operation before we could complete it
-                delegate.onResponse(null);
+                delegate.onResponse(OptionalBytesReference.MISSING);
             } else {
                 delegate.onFailure(e);
             }

x-pack/plugin/snapshot-repo-test-kit/qa/s3/src/javaRestTest/java/org/elasticsearch/repositories/blobstore/testkit/S3SnapshotRepoTestKitIT.java

@@ -28,10 +28,4 @@ protected Settings repositorySettings() {
 
         return Settings.builder().put("client", "repo_test_kit").put("bucket", bucket).put("base_path", basePath).build();
     }
-
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/96162")
-    @Override
-    public void testRepositoryAnalysis() throws Exception {
-        super.testRepositoryAnalysis();
-    }
 }