Cleanup Concurrent RepositoryData Loading (elastic#48329)

The loading of `RepositoryData` is not an atomic operation.
It uses a list + get combination of calls.
This lead to accidentally returning an empty repository data
for generations >=0 which can never not exist unless the repository
is corrupted.
In the test elastic#48122 (and other SLM tests) there was a low chance of
running into this concurrent modification scenario and the repository
actually moving two index generations between listing out the
index-N and loading the latest version of it. Since we only keep
two index-N around at a time this lead to unexpectedly absent
snapshots in status APIs.
Fixing the behavior to be more resilient is non-trivial but in the works.
For now I think we should simply throw in this scenario. This will also
help prevent corruption in the unlikely event but possible of running into this
issue in a snapshot create or delete operation on master failover on a
repository like S3 which doesn't have the "no overwrites" protection on
writing a new index-N.

Fixes elastic#48122

server/src/main/java/org/elasticsearch/repositories/blobstore/BlobStoreRepository.java

@@ -650,6 +650,9 @@ public void endVerification(String seed) {
     public RepositoryData getRepositoryData() {
         try {
             final long indexGen = latestIndexBlobId();
+            if (indexGen == RepositoryData.EMPTY_REPO_GEN) {
+                return RepositoryData.EMPTY;
+            }
             final String snapshotsIndexBlobName = INDEX_FILE_PREFIX + Long.toString(indexGen);
 
             RepositoryData repositoryData;
@@ -688,9 +691,6 @@ public RepositoryData getRepositoryData() {
                 }
             }
             return repositoryData;
-        } catch (NoSuchFileException ex) {
-            // repository doesn't have an index blob, its a new blank repo
-            return RepositoryData.EMPTY;
         } catch (IOException ioe) {
             throw new RepositoryException(metadata.name(), "could not read repository data from index blob", ioe);
         }