refactor(Core): apply clang-tidy modernize-use-equals-default (#3834)

src/server/game/Grids/Cells/Cell.h

@@ -17,7 +17,7 @@ class WorldObject;
 
 struct CellArea
 {
-    CellArea() {}
+    CellArea() = default;
     CellArea(CellCoord low, CellCoord high) : low_bound(low), high_bound(high) {}
 
     bool operator!() const { return low_bound == high_bound; }

src/server/game/Grids/Grid.h

@@ -40,7 +40,7 @@ public:
     /** destructor to clean up its resources. This includes unloading the
     grid if it has not been unload.
     */
-    ~Grid() {}
+    ~Grid() = default;
 
     /** an object of interested enters the grid
      */