Fix contest creation

app/models/contest.rb

@@ -39,12 +39,12 @@ class Contest < ApplicationRecord
 
   friendly_id :name, use: :slugged
 
-  before_save :add_duration_seconds
+  before_save :add_duration_seconds, if: -> { duration.present? }
 
   validates :name, presence: true
   validates :lang, inclusion: { in: Languages::AVAILABLE_LANGUAGES.map { |lang| lang[:id] } }
   validates :ranking_mode, inclusion: { in: Ranking::AVAILABLE_RANKING_MODES.map { |lang| lang[:id] } }
-  validates :duration, format: { with: /\A(\d\d:\d\d|\d:\d\d)\z/ }
+  validates :duration, format: { with: /\A(\d\d:\d\d|\d:\d\d)\z/ }, if: -> { duration.present? }
 
   generates_token_for :token