fix(SUP-40028): The playlist player freezes during the playback

[Tzipi-kaltura]

Description of the Changes

issue:
the playlist player freezes when moving to youtube entry (when timeline plugin is loaded).

solution:
youtube doesn't have getThumbnail function, adding validation.

solves SUP-40028

CheckLists

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• test are passing in local environment
• Travis tests are passing (or test results are not worse than on master branch :))
• Docs have been updated

[JonathanTGold]

This is incorrect
YouTube engine should implement the IEngine interface (can simply return null)
And will address the root of the issue instead of adding a patch