Data Node Dashboard, error in widgets : Cannot invoke"org.graylog.shaded.opensearch2.org.opensearch.search.aggregations.Aggregations.get(String)"

## Expected Behavior
The dashboard must load data

## Current Behavior
All the widgets contains :
`Cannot invoke "org.graylog.shaded.opensearch2.org.opensearch.search.aggregations.Aggregations.get(String)" because the return value of "org.graylog.shaded.opensearch2.org.opensearch.search.aggregations.bucket.MultiBucketsAggregation$Bucket.getAggregations()" is null.`

In server.log :
```
2025-12-03T11:52:34.607+01:00 WARN [OpenSearchBackend] Unable to extract results:
org.graylog.plugins.views.search.errors.SearchException: Cannot invoke "org.graylog.shaded.opensearch2.org.opensearch.search.aggregations.Aggregations.get(String)" because the return value of "org.graylog.shaded.opensearch2.org.opensearch.search.aggregations.bucket.MultiBucketsAggregation$Bucket.getAggregations()" is null.
at org.graylog.plugins.views.search.engine.SearchTypeHandler.extractResult(SearchTypeHandler.java:57)
at org.graylog.storage.opensearch2.views.OpenSearchBackend.doRun(OpenSearchBackend.java:318)
at org.graylog.storage.opensearch2.views.OpenSearchBackend.doRun(OpenSearchBackend.java:82)
at org.graylog.plugins.views.search.engine.QueryBackend.run(QueryBackend.java:94)
at org.graylog.plugins.views.search.engine.QueryEngine.prepareAndRun(QueryEngine.java:162)
at org.graylog.plugins.views.search.engine.QueryEngine.lambda$execute$5(QueryEngine.java:131)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)
```

## Steps to Reproduce (for bugs)
1. Install Graylog Enterprise 7
2. Go into System > Cluster Configuration > Data Node Dashboard

## Your Environment

* Graylog Version: graylog-enterprise-7.0.1-1.x86_64
* Java Version: java-21-openjdk-headless-21.0.9.0.10-1.el9.x86_64
* OpenSearch Version: graylog-datanode-7.0.1-1.x86_64
* MongoDB Version: mongodb-org-7.0.26-1.el9.x86_64
* Operating System: Rocky Linux release 9.6 (Blue Onyx)
* Browser version: Firefox 146

## Checklist
[x] This issue fix need to be backported.