Prevent access of invalid references

[joaopaulovieira]

Summary

Some calls to this.shakaPlayerInstance.getManifest() are doing before one valid shaka player instance exists. This PR prevents those access to avoid log noises.

Changes

• Returns null on presentationTimeline getter if this.shakaPlayerInstance.getManifest() doesn't exists;
• Returns 0 on getProgramDateTime method if this.presentationTimeline returns invalid value;

How to test

• Play any media;
• Open console tab of your developers tools;
  • Should not have any console about access presentationTimeline of null.

A picture is worth a thousand words

Before

Screen.Recording.2021-01-06.at.10.27.31.mov

After

Screen.Recording.2021-01-06.at.10.24.51.mov