fix: DrmEngine exception thrown when using FairPlay

[martinstark]

Closes #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. I opened this PR more as a point of discussion around the issue at hand.

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.

shaka-player/lib/media/drm_engine.js

Lines 483 to 498 in c471d23

	const manifestInitData = this.currentDrmInfo_.initData.find(
	(initDataOverride) => initDataOverride.initData.length > 0);
	/**
	* We can attach media keys before the playback actually begins when:
	* - If we are not using FairPlay Modern EME
	* - Some initData already has been generated (through the manifest)
	* - In case of an offline session
	*/
	if (manifestInitData ||
	this.currentDrmInfo_.keySystem !== 'com.apple.fps' ||
	this.offlineSessionIds_.length) {
	await this.attachMediaKeys_();
	}
	this.createOrLoad();

shaka-player/lib/media/drm_engine.js

Lines 626 to 629 in c471d23

	createOrLoad() {
	// Create temp sessions.
	const initDatas =
	(this.currentDrmInfo_ ? this.currentDrmInfo_.initData : []) || [];

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Incremental code coverage: 100.00%