[Synonym] Return Empty RuleSet

[Samiul-TheSoccerFan]

This PR fixes an existing issue where GET _synonyms does not include ruleset with empty/no rules.

Setup

PUT _synonyms/ruleset-without-rules
{
  "synonyms_set": []
}

PUT _synonyms/ruleset-with-rules
{
  "synonyms_set": [
      {
        "id": "search_terms",
        "synonyms": "search,find,lookup,query => search"
      },
      {
        "id": "technology_terms", 
        "synonyms": "laptop,notebook,computer,pc"
      },
      {
        "id": "clothing_terms",
        "synonyms": "shirt,top,tee => shirt"
      },
      {
        "id": "vehicle_terms",
        "synonyms": "car,auto,automobile,vehicle"
      },
      {
        "id": "happy_emotions",
        "synonyms": "happy,joyful,cheerful,glad,pleased"
      }
    ]
  }
}

GET _synonyms //  return both

[elasticsearchmachine]

Hi @Samiul-TheSoccerFan, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/search-eng (Team:SearchOrg)

[elasticsearchmachine]

Pinging @elastic/search-relevance (Team:Search - Relevance)

[carlosdelest]

Nice work @Samiul-TheSoccerFan ! Thanks for fixing this bug 🐛 , changes LGTM.

You'll need to add the new test feature to other YAML test suites - check #129766 for an example of adding the test feature to other YAML tests. Apply this to all the YAML tests suites that error out

[carlosdelest]

LGTM, thanks @Samiul-TheSoccerFan !

I added some comments about fixing the CI failures.

[Samiul-TheSoccerFan]

Thank you @carlosdelest for the review. One note related to the CI failures, I added the new FeatureSpecification (see commit) as per CI logs. Do we still need to add a FeatureFlag or this should be enough?

[kderusso]

LGTM! Nice work. I'm not sure if we have to add a feature flag @carlosdelest per your example?

[Samiul-TheSoccerFan]

@elasticmachine update branch

[carlosdelest]

You'll need to add the new test feature to other YAML test suites - check #129766

@Samiul-TheSoccerFan @kderusso I'm sorry, I threw you on the wrong path - there is no need to implement the code I mentioned, it is enough with what is implemented here. I saw the errors and automatically remembered that, should have double checked.

[Samiul-TheSoccerFan]

Thank you @carlosdelest for the confirmation :)

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	9.1
✅	8.19
✅	9.0
✅	8.18