API: Fix out of bound error on empty playlists

[Fijxu]

Invidious assumes that every playlist will have at least one video because it needs to check for the index key. So if there is no videos on a playlist, there is no index key and Invidious throws Index out of bounds

This fixes this API endpoints when there is no videos in a playlist:

api/v1/playlists/:plid
api/v1/auth/playlists/:plid

For some weird reason, the api/v1/auth/playlists/:plid endpoint was able the return the error Index out of bounds as said in #4679 but api/v1/playlists/:plid did not return anything. Weird

Fixes #4679

[Fijxu]

Can be tested with a empty playlist that I created at https://debuginv.nadeko.net/api/v1/playlists/PL5ASK6Z87scO0GrGbMzxVz5qvMhRUitbG

[SamantazFox]

For some weird reason, the api/v1/auth/playlists/:plid endpoint was able the return the error Index out of bounds as said in #4679 but api/v1/playlists/:plid did not return anything. Weird

This because the authentication middleware (AuthHandler) catches the error and displays it nicely:

invidious/src/invidious/helpers/handlers.cr

Lines 127 to 133 in bad9209

	rescue ex
	env.response.content_type = "application/json"
	error_message = {"error" => ex.message}.to_json
	env.response.status_code = 403
	env.response.print error_message
	end

[SamantazFox]

Thank you for fixing this :)