Handle missing duration for video premieres

[marcoroth]

The YouTube API omits contentDetails.duration while a video is an ongoing premiere. The default value 0 was then passed to to_seconds, which expects an ISO 8601 string, raising undefined method 'match' for an instance of Integer:

duration error: NoMethodError: undefined method 'match' for an instance of Integer
  /Users/marcoroth/.local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/yt-0.34.1/lib/yt/models/content_detail.rb:62:in 'Yt::Models::ContentDetail#to_seconds'
  /Users/marcoroth/.local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/yt-0.34.1/lib/yt/models/content_detail.rb:18:in 'block in <class:ContentDetail>'

This pull request fixes this so that Video#duration now returns 0 and Video#length "00:00:00" in that case.