fix sonar

Author: hjgraca

libraries/src/AWS.Lambda.Powertools.Metrics/Model/MetricDirective.cs

@@ -121,7 +121,9 @@ public List<string> AllDimensionKeys
 
             defaultKeys.AddRange(keys);
 
-            return defaultKeys.Any() ? defaultKeys : new List<string>();
+            if (defaultKeys.Count == 0) defaultKeys = new List<string>();
+
+            return defaultKeys;
         }
     }