@ngrx/effects: testing an effect returns empty()

The problem is that you are comparing the actual result against cold('|').

The pipe character in cold('|') represents the completion of the observable stream. However, your effect will not complete. The empty() observable does complete, but returning empty() from switchMap just sees that observable merged into the effect observable's stream - it does not complete the effect observable.

Instead, you should compare the actual result against cold('') - an observable that emits no values and does not complete.

The best way to do this is to use dispatch false in the effect and change the swicthMap for a tap

@Effect({dispatch: false}) showDialog$ = this.actions$.pipe(
   ofType( ActionTypes.DIALOG_SHOW ),
   map( ( action: DialogAction ) => action.payload ),
   tap( payload => 
      this.dialogsService.showDialog( payload.className )
    ));

To test it you can do like

describe( 'showDialog$', () => {
    it( 'should return an empty observable', () => {
        const dialogName = 'testDialog';
        const action = showDialog( dialogName );
        actions = hot( '--a-', { a: action } );

       expect(effects.showDialog$).toBeObservable(actions);
// here use mock framework to check the dialogService.showDialog is called
    } );
} );

Notice the reason I use toBeObservable(actions) thats because due to disptach false this effect will not produce anything so actions is the same as before , what you gain by calling toBeObservable(actions) is that is makes the test synchronous, so you can check after it that your service was called with a mock framework like jasmine or ts-mockito