[sample_logs] don't filter out empty string, only nulls (#1315)

Partially fixes 1 panel. I don't uncomment the test, as another fix is
needed for it to fully work.

Author: trzysiek

platform/parsers/elastic_query_dsl/pancake_json_rendering.go

@@ -159,7 +159,7 @@ func (p *pancakeJSONRenderer) potentiallyRemoveExtraBucket(layer *pancakeModelLa
 		for i, row := range bucketRows {
 			for _, col := range row.Cols {
 				if strings.HasPrefix(col.ColName, bucketKeyName) {
-					if col.Value == nil || col.Value == "" { // TODO: replace with schema
+					if col.Value == nil {
 						nullRowToDelete = i
 						break ROW
 					}