### Elasticsearch Version

8.3.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

arm64

### Problem Description

When using the `_terms_enum` API, if remote clusters return failures, but the local cluster doesn't, a null exception is thrown:

```
{
  "error": {
    "root_cause": [
      {
        "type": "null_pointer_exception",
        "reason": "Cannot invoke \"java.util.List.add(Object)\" because \"shardFailures\" is null"
      }
    ],
    "type": "null_pointer_exception",
    "reason": "Cannot invoke \"java.util.List.add(Object)\" because \"shardFailures\" is null"
  },
  "status": 500
}
```

### Steps to Reproduce

Call the _terms_enum API with a remote cluster that will return failures, e.g. because of this bug: https://github.com/elastic/elasticsearch/issues/86187

### Logs (if relevant)

_No response_
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/termsenum/action/TransportTermsEnumAction.java
atomicResponses.set(opsIndex, response);
            if (expectedOps == counterOps.incrementAndGet()) {
                finishHim(true);
            } else {
                checkForEarlyFinish();
            }
        }
        private void onNodeFailure(String nodeId, int opsIndex, Exception exc) {
            logger.trace("received failure {} for node {}", exc, nodeId);
            
            if (expectedOps == counterOps.incrementAndGet()) {
                finishHim(true);
            }
        }
        private void onRemoteClusterFailure(String clusterAlias, int opsIndex, Exception exc) {
            logger.trace("received failure {} for cluster {}", exc, clusterAlias);
            
            if (expectedOps == counterOps.incrementAndGet()) {
                finishHim(true);
            }
        }
        
        protected synchronized void finishHim(boolean complete) {
            if (listener == null) {
                return;
            }
            try {
                listener.onResponse(mergeResponses(request, atomicResponses, complete, nodeBundles));
            } catch (Exception e) {
                listener.onFailure(e);