fix: dereference of possibly null references in core graphics (#2377)

- Null Check: RenderingEntities == null checks if RenderingEntities is not initialized.
- Null Check: Globals.MapGrid == null checks if the 2D array is not initialized.
- Removed default Check: Globals.MapGrid is an array, checking against default is unnecessary.

Intersect.Client/Core/Graphics.cs

@@ -241,7 +241,7 @@ public static void DrawInGame(TimeSpan deltaTime)
             return;
         }
 
-        if (Globals.NeedsMaps || Globals.MapGrid == default)
+        if (Globals.NeedsMaps || Globals.MapGrid == null || RenderingEntities == null)
         {
             return;
         }