[Maps] Fix TS error in ESGeoGridSource by adding array check for compositeSplit buckets (#244740)

Resolves partially #187897

Author: olapawlus

x-pack/platform/plugins/shared/maps/public/classes/sources/es_geo_grid_source/es_geo_grid_source.tsx

@@ -383,8 +383,8 @@ export class ESGeoGridSource extends AbstractESAggSource implements IMvtVectorSo
 
       const aggr = esResponse.aggregations?.compositeSplit as AggregationsCompositeAggregate;
       afterKey = aggr.after_key;
-      // @ts-expect-error upgrade typescript v5.1.6
-      if (aggr.buckets.length < gridsPerRequest) {
+
+      if (Array.isArray(aggr.buckets) && aggr.buckets.length < gridsPerRequest) {
         // Finished because request did not get full resultset back
         break;
       }