[Stack Monitoring] Empty include array in terms aggregation fails to retrieve logstash pipelines

[consulthys]

Describe the bug:

In Stack Monitoring, when retrieving Logstash pipelines to be displayed on the Logstash > Pipelines page, the aggregation query might contain an empty "include": [] array, which has the nasty effect of... excluding everything

"by_pipeline_id": {
	"terms": {
		"field": "logstash.node.stats.pipelines.id",
		"size": 1000,
		"include": []                 <---------- Culprit
	},

Steps to reproduce:

The issue can easily be reproduced with the following test:

Test

POST test/_bulk
{"index":{}}
{"test": "foo"}
{"index":{}}
{"test": "bar"}

# returns both keywords
POST test/_search
{
  "size": 0,
  "aggs": {
    "fields": {
      "terms": {
        "field": "test.keyword"
      }
    }
  }
}

# returns no keywords
POST test/_search
{
  "size": 0,
  "aggs": {
    "fields": {
      "terms": {
        "field": "test.keyword",
        "include": []
      }
    }
  }
}

Expected behavior:

If the include array is empty, it should not be included at all in the terms aggregation query.

"by_pipeline_id": {
	"terms": {
		"field": "logstash.node.stats.pipelines.id",
		"size": 1000
	},