Threshold based alert throws exception when no documents are returned

[spinscale]

Kibana version: 7.16.3

Elasticsearch version: 7.16.3

Describe the bug:

When creating a threshold based alert (via watcher) an exception is thrown when a null value is returned by an aggregation due to not matching any documents

Steps to reproduce:

Run this in kibana console

PUT test/_doc/1
{
  "value" : 123
}

# comment this out to see everything working
DELETE test/_doc/1

POST _watcher/watch/_execute
{
  "watch": {
    "trigger": {
      "schedule": {
        "interval": "10h"
      }
    },
    "input": {
      "search": {
        "request": {
          "search_type": "query_then_fetch",
          "indices": [
            "test"
          ],
          "rest_total_hits_as_int": true,
          "body": {
            "size": 0,
            "aggs": {
              "metricAgg": {
                "avg": {
                  "field": "value"
                }
              }
            }
          }
        }
      }
    },
    "condition": {
      "script": {
        "source": "if (ctx.payload.aggregations.metricAgg.value > params.threshold) { return true; } return false;",
        "lang": "painless",
        "params": {
          "threshold": 1.2
        }
      }
    },
    "actions": {
      "logme": {
        "logging": {
          "text": "{{ctx}}"
        }
      }
    }
  }
}

Expected behavior:

No exception is thrown, as the exception should cater for this behaviour.

Fix would be to check for null in the condition (that is created by kibana for threshold based alerts).

if (ctx.payload.aggregations.metricAgg.value != null && ctx.payload.aggregations.metricAgg.value > params.threshold) { return true; } return false;

Additional information: https://discuss.elastic.co/t/watcher-execution-history-error-state/297139/2

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)