### 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/mapper-constant-keyword/src/main/java/org/elasticsearch/xpack/constantkeyword/mapper/ConstantKeywordFieldMapper.java
private final String value;
        public ConstantKeywordFieldType(String name, String value, Map<String, String> meta) {
            super(name, meta);
            this.value = value;
        }
        public ConstantKeywordFieldType(String name, String value) {
            this(name, value, Collections.emptyMap());
        }
        
        public String value() {
            return value;
        }
        @Override
        public String typeName() {
            return CONTENT_TYPE;
        }
        @Override
        public String familyTypeName() {
            return KeywordFieldMapper.CONTENT_TYPE;
        }
        @Override
        public IndexFieldData.Builder fielddataBuilder(String fullyQualifiedIndexName, Supplier<SearchLookup> searchLookup) {
            return new ConstantIndexFieldData.Builder(
                value,