fix(ui): when session is complete, null out progress image

This may cause minor gallery jumpiness at the very end of processing, but is necessary to prevent the progress image from sticking around if the last node in a session did not have an image output.

invokeai/frontend/web/src/features/system/store/systemSlice.ts

@@ -348,6 +348,7 @@ export const systemSlice = createSlice({
       state.currentStep = 0;
       state.totalSteps = 0;
       state.statusTranslationKey = 'common.statusConnected';
+      state.progressImage = null;
     });
 
     /**