Backends: fixed reappearing uses to NULL to favor nullptr.

backends/imgui_impl_opengl3.h

@@ -40,7 +40,7 @@ IMGUI_IMPL_API void     ImGui_ImplOpenGL3_RenderDrawData(ImDrawData* draw_data);
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateDeviceObjects();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyDeviceObjects();
 
-// (Advanced) Use e.g. if you need to precisely control the timing of texture updates (e.g. for staged rendering), by setting ImDrawData::Textures = NULL to handle this manually.
+// (Advanced) Use e.g. if you need to precisely control the timing of texture updates (e.g. for staged rendering), by setting ImDrawData::Textures = nullptr to handle this manually.
 IMGUI_IMPL_API void     ImGui_ImplOpenGL3_UpdateTexture(ImTextureData* tex);
 
 // Configuration flags to add in your imconfig file: