fix: Set default key to 'UNKNOWN' for empty bucket keys.

Author: mjabascal10

backend/src/main/java/com/park/utmstack/util/chart_builder/elasticsearch_dsl/responses/impl/metric_chart/ResponseParserForMetricChart.java

@@ -81,6 +81,9 @@ private List<MetricChartResult> parseTermAggregation(Map<String, Aggregate> resu
 
             for (Metric metric : metrics) {
                 for (BucketAggregation _bucket : _buckets) {
+                    if(_bucket.getKey().isEmpty()){
+                        _bucket.setKey("UNKNOWN");
+                    }
                     switch (metric.getAggregation()) {
                         case AVERAGE:
                             AvgAggregate avg = _bucket.getSubAggregations().get(metric.getId()).avg();