chore(core): tiny perf improvement in MapUtils.merge()

MapUtils.merge() un-necessary clone the map when there is only one map that is not-null and not-empty which is not needed as the map is not modified but returned immediatly.

core/src/main/java/io/kestra/core/utils/MapUtils.java

@@ -18,11 +18,11 @@ public static Map<String, Object> merge(Map<String, Object> a, Map<String, Objec
         }
 
         if (a == null || a.isEmpty()) {
-            return copyMap(b);
+            return b;
         }
 
         if (b == null || b.isEmpty()) {
-            return copyMap(a);
+            return a;
         }
 
         Map copy = copyMap(a);