fix(animations): ensure a null easing value is never used with web-animations

Closes #9780
Closes #9752

modules/@angular/platform-browser/src/dom/web_animations_driver.ts

@@ -48,13 +48,18 @@ export class WebAnimationsDriver implements AnimationDriver {
       formattedSteps = [start, start];
     }
 
-    var playerOptions = {
+    var playerOptions: {[key: string]: string | number} = {
       'duration': duration,
       'delay': delay,
-      'easing': easing,
       'fill': 'both'  // we use `both` because it allows for styling at 0% to work with `delay`
     };
 
+    // we check for this to avoid having a null|undefined value be present
+    // for the easing (which results in an error for certain browsers #9752)
+    if (easing) {
+      playerOptions['easing'] = easing;
+    }
+
     var player = this._triggerWebAnimation(anyElm, formattedSteps, playerOptions);
 
     return new WebAnimationsPlayer(player, duration);