### 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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.constantkeyword.mapper;
import org.apache.lucene.index.TermsEnum;
import org.apache.lucene.search.MatchAllDocsQuery;
import org.apache.lucene.search.MatchNoDocsQuery;
import org.apache.lucene.search.MultiTermQuery;
import org.apache.lucene.search.Query;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.UnicodeUtil;
import org.apache.lucene.util.automaton.Automaton;
import org.apache.lucene.util.automaton.CharacterRunAutomaton;
import org.apache.lucene.util.automaton.LevenshteinAutomata;
import org.apache.lucene.util.automaton.RegExp;
import org.elasticsearch.common.geo.ShapeRelation;
import org.elasticsearch.common.lucene.BytesRefs;
import org.elasticsearch.common.regex.Regex;
import org.elasticsearch.common.time.DateMathParser;
import org.elasticsearch.common.unit.Fuzziness;
import org.elasticsearch.index.fielddata.FieldData;