Search - fixed Nullpointer in significant text agg when field does not exist

[markharwood]

Backport (#64144)

Fixed Nullpointer in significant text agg - added test
Closes #64045

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[zetaab]

@markharwood this change will break our production currently when using 7.11.1. You fixed nullpointer issue, but at the same time you broke up another "feature" which was that you could still execute query if the field did not exist.

For instance when we query it says now

  "reason": "Field [q14016] does not exist, SignificantText requires an analyzed field"

So instead of just querying like we did before, now we need first list index mapping and check does the fields exists. Then execute another query where we search stuff if the field did exists. I have not found way to make it work without that.

{
  "query": {
    "exists": {
      "field": "q14016"
    }
  },
  "aggs": {
    "keywords": {
      "significant_text": {
        "field": "q14016"
      }
    }
  }
}

for instance this returns same result (error)

[markharwood]

Thanks for reporting @zetaab I opened this issue #69809

For now using a query or parent filter aggregation with an exists query is a workaround