fix(Ads): Avoid adding custom interstitials without URI

lib/ads/interstitial_ad_manager.js

@@ -404,10 +404,18 @@ shaka.ads.InterstitialAdManager = class {
   addInterstitials(interstitials) {
     let cuepointsChanged = false;
     for (const interstitial of interstitials) {
+      if (!interstitial.uri) {
+        shaka.log.alwaysWarn('Missing URL in interstitial', interstitial);
+        continue;
+      }
       const interstitialId = interstitial.id || JSON.stringify(interstitial);
       if (this.interstitialIds_.has(interstitialId)) {
         continue;
       }
+      if (interstitial.loop && !interstitial.overlay) {
+        shaka.log.alwaysWarn('Loop is only supported in overlay interstitials',
+            interstitial);
+      }
       if (!interstitial.overlay) {
         cuepointsChanged = true;
       }

test/ads/interstitial_ad_manager_unit.js

@@ -953,6 +953,32 @@ describe('Interstitial Ad manager', () => {
       expect(onEventSpy).toHaveBeenCalledWith(
           jasmine.objectContaining(eventValue1));
     });
+
+    it('ignore invalid interstitial', async () => {
+      // It is not valid because it does not have an interstitial URL
+      const interstitial = {
+        id: null,
+        startTime: 10,
+        endTime: null,
+        uri: '',
+        mimeType: null,
+        isSkippable: true,
+        skipOffset: 10,
+        skipFor: null,
+        canJump: false,
+        resumeOffset: null,
+        playoutLimit: null,
+        once: true,
+        pre: false,
+        post: false,
+        timelineRange: false,
+        loop: false,
+        overlay: null,
+      };
+      await interstitialAdManager.addInterstitials([interstitial]);
+
+      expect(onEventSpy).not.toHaveBeenCalled();
+    });
   });
 
   describe('VAST', () => {