optional fields in term vector res (#2724)

(cherry picked from commit 5615a46)

Author: l-trotta

output/openapi/elasticsearch-openapi.json

@@ -94,7 +94,7 @@ export class Operation {
 }
 
 export class TermVectorsResult {
-  _id: Id
+  _id?: Id
   _index: IndexName
   _version?: VersionNumber
   took?: long

output/openapi/elasticsearch-serverless-openapi.json

@@ -25,7 +25,7 @@ import { TermVector } from './types'
 export class Response {
   body: {
     found: boolean
-    _id: Id
+    _id?: Id
     _index: IndexName
     term_vectors?: Dictionary<Field, TermVector>
     took: long

output/schema/schema-serverless.json

@@ -21,7 +21,7 @@ import { Dictionary } from '@spec_utils/Dictionary'
 import { double, integer, long } from '@_types/Numeric'
 
 export class TermVector {
-  field_statistics: FieldStatistics
+  field_statistics?: FieldStatistics
   terms: Dictionary<string, Term>
 }