[EEM] Inconsistent return type for service fields

[kpatticha]

When the value is not present, it returns an empty array

"service": {
  "environment": [],
    "name": "without-env-2",
    "namespace": [],
    "runtime": {
      "name": [],
      "version": []
    },
    "language": {
      "name": []
    },
    "version": []
},

when it's present it returns a string

"service": {
  "environment": "Synthtrace: simple_trace",
  "name": "synth-node-2",
  "namespace": [],
  "runtime": {
    "name": [],
    "version": []
  },
  "language": {
    "name": []
  },
  "version": []
},

related

https://github.com/elastic/kibana/blob/b3f9721f8ed505f8f40d7c6addc4b7e913d3d387/x-pack/plugins/observability_solution/apm/server/routes/entities/get_entities.ts#L76-L78

[elasticmachine]

Pinging @elastic/obs-knowledge-team (Team:obs-knowledge)

[klacabane]

looks like duplicate of https://github.com/elastic/elastic-entity-model/issues/118