fix(PlayerMicroformat): Make the embed field optional

[absidue]

Description

Videos like https://www.youtube.com/watch?v=HQMkxmtPZhg don't have the embed property in the player microformat response, which causes YouTube.js to log an error about not being able to access iframeUrl on undefined.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings