fix: check that changes are defined

It can easily happen that items are undefined as a change when
ngOnChanges is called in GalleryComponent. This is a deficiency of the
Angular's types that this is not checked, and it slipped through my test
suite as well.

So I created a stricter type that will keep the possibility of changes
being undefined in check and reorganized the test so that the ngOnChanges
with undefined `items` change is checked as well.

libs/gallery/src/lib/components/gallery/gallery.component.spec.ts

@@ -36,6 +36,8 @@ describe('GalleryComponent', () => {
 
   it('should handle empty items input in looping mode', () => {
     componentRef.setInput('items', undefined);
+    fixture.detectChanges();
+
     componentRef.setInput('loop', true);
     fixture.detectChanges();
 

libs/gallery/src/lib/components/gallery/gallery.component.ts

@@ -9,7 +9,6 @@ import {
   Input,
   OnChanges,
   Output,
-  SimpleChanges,
   TemplateRef,
   ViewChild,
 } from '@angular/core';
@@ -27,6 +26,7 @@ import {
   VerticalOrientation,
 } from '../../core';
 import { defaultAria } from '../../core/aria';
+import { StrictSimpleChanges } from '../../core/ng';
 import { ThumbsComponent } from '../thumbs/thumbs.component';
 import { ViewerComponent } from '../viewer/viewer.component';
 
@@ -191,8 +191,8 @@ export class GalleryComponent implements OnChanges {
       : OrientationFlag.VERTICAL;
   }
 
-  ngOnChanges({ items }: SimpleChanges) {
-    if (!items.currentValue) {
+  ngOnChanges({ items }: StrictSimpleChanges<GalleryComponent>) {
+    if (!items?.currentValue) {
       this.items = [];
     }
   }

libs/gallery/src/lib/core/ng.ts

@@ -1,6 +1,10 @@
-import { ComponentRef } from '@angular/core';
+import { ComponentRef, SimpleChange, SimpleChanges } from '@angular/core';
 
 export interface StrictComponentRef<C> extends ComponentRef<C> {
   // `& string` because otherwise keyof would return string | number
   setInput(name: keyof C & string, value: unknown): void;
 }
+
+export type StrictSimpleChanges<T> = SimpleChanges & {
+  [propName in keyof T]: SimpleChange | undefined;
+};