fix: DrmEngine exception thrown when using FairPlay (shaka-project#4971)

Closes shaka-project#4902

This is not intended to be a complete fix for the issue, I do not
understand the effect this new ternary on the shaka player as a whole. I
have not found the root cause for `this.currentDrmInfo_` being
`undefined` when accessed. It does, however, look like it has previously
been acceptable for this property to be `undefined`.

During the work to add Fairplay support in 2022, accessing
`this.currentDrmInfo_` was added on line 483. This line **does not**
accept that `this.currentDrmInfo_` can be undefined. A few lines below
this, `this.createOrLoad()` is called, which **does** accept that
`this.currentDrmInfo_` can be `undefined`. The latter piece of code was
last modified in 2019.

https://github.com/shaka-project/shaka-player/blob/c471d23bc25db11dda85a18870ebd3fe37971848/lib/media/drm_engine.js#L483-L498

https://github.com/shaka-project/shaka-player/blob/c471d23bc25db11dda85a18870ebd3fe37971848/lib/media/drm_engine.js#L626-L629

lib/media/drm_engine.js

@@ -480,8 +480,10 @@ shaka.media.DrmEngine = class {
           () => this.closeOpenSessions_());
     }
 
-    const manifestInitData = this.currentDrmInfo_.initData.find(
-        (initDataOverride) => initDataOverride.initData.length > 0);
+    const manifestInitData = this.currentDrmInfo_ ?
+      this.currentDrmInfo_.initData.find(
+          (initDataOverride) => initDataOverride.initData.length > 0,
+      ) : null;
 
     /**
      * We can attach media keys before the playback actually begins when: