Initialize CommonStatsFlags with empty array for levels

Signed-off-by: Peter Alfonsi <petealft@amazon.com>

server/src/main/java/org/opensearch/action/admin/indices/stats/CommonStatsFlags.java

@@ -66,7 +66,7 @@ public class CommonStatsFlags implements Writeable, Cloneable {
     private boolean includeOnlyTopIndexingPressureMetrics = false;
     // Used for metric CACHE_STATS, to determine which caches to report stats for
     private EnumSet<CacheType> includeCaches = EnumSet.noneOf(CacheType.class);
-    private String[] levels;
+    private String[] levels = new String[0];
 
     /**
      * @param flags flags to set. If no flags are supplied, default flags will be set.
@@ -139,7 +139,7 @@ public CommonStatsFlags all() {
         includeAllShardIndexingPressureTrackers = false;
         includeOnlyTopIndexingPressureMetrics = false;
         includeCaches = EnumSet.noneOf(CacheType.class);
-        levels = null;
+        levels = new String[0];
         return this;
     }
 
@@ -156,7 +156,7 @@ public CommonStatsFlags clear() {
         includeAllShardIndexingPressureTrackers = false;
         includeOnlyTopIndexingPressureMetrics = false;
         includeCaches = EnumSet.noneOf(CacheType.class);
-        levels = null;
+        levels = new String[0];
         return this;
     }