### 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
private final ClusterService clusterService;
    private final TransportService transportService;
    private final RemoteClusterService remoteClusterService;
    private final SearchService searchService;
    private final IndicesService indicesService;
    private final ScriptService scriptService;
    private final IndexNameExpressionResolver indexNameExpressionResolver;
    final String transportShardAction;
    private final String shardExecutor;
    private final XPackLicenseState licenseState;