Backends: fixed reappearing uses to NULL to favor nullptr.

backends/imgui_impl_sdl2.cpp

@@ -684,7 +684,7 @@ static void ImGui_ImplSDL2_UpdateMouseData()
         // Note that SDL_GetGlobalMouseState() is in theory slow on X11, but this only runs on rather specific cases. If a problem we may provide a way to opt-out this feature.
         SDL_Window* hovered_window = SDL_GetMouseFocus();
         const bool is_relative_mouse_mode = SDL_GetRelativeMouseMode() != 0;
-        if (hovered_window == NULL && bd->MouseCanUseGlobalState && bd->MouseButtonsDown == 0 && !is_relative_mouse_mode)
+        if (hovered_window == nullptr && bd->MouseCanUseGlobalState && bd->MouseButtonsDown == 0 && !is_relative_mouse_mode)
         {
             // Single-viewport mode: mouse position in client window coordinates (io.MousePos is (0,0) when the mouse is on the upper-left corner of the app window)
             int mouse_x, mouse_y;