### 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
return new MatchNoDocsQuery();
            }
        }
        @Override
        public Query regexpQuery(
            String regexp,
            int syntaxFlags,
            int matchFlags,
            int maxDeterminizedStates,
            MultiTermQuery.RewriteMethod method,
            SearchExecutionContext context
        ) {
            if (this.value == null) {
                return new MatchNoDocsQuery();
            }
            final Automaton automaton = new RegExp(regexp, syntaxFlags, matchFlags).toAutomaton(maxDeterminizedStates);
            final CharacterRunAutomaton runAutomaton = new CharacterRunAutomaton(automaton);
            if (runAutomaton.run(this.value)) {
                return new MatchAllDocsQuery();
            } else {
                return new MatchNoDocsQuery();
            }
        }
    }
    ConstantKeywordFieldMapper(String simpleName, MappedFieldType mappedFieldType) {
        super(simpleName, mappedFieldType, MultiFields.empty(), CopyTo.empty());
    }
    @Override
    public ConstantKeywordFieldType fieldType() {
        return (ConstantKeywordFieldType) super.fieldType();