### 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 Iterator<String> iterator;
        private String current;
        private TermIterator(Iterator<String> iterator) {
            this.iterator = iterator;
            this.current = iterator.next();
        }
        public String term() {
            return current;
        }
        @Override
        public boolean hasNext() {
            return iterator.hasNext();
        }
        @Override
        public String next() {
            return current = iterator.next();
        }
        @Override
        public int compareTo(TermIterator o) {
            return current.compareTo(o.term());
        }
    }
}