Use ENV.fetch for ffmpeg/ffprobe defaults (#35081)

config/initializers/ffmpeg.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
 Rails.application.configure do
-  config.x.ffmpeg_binary = ENV['FFMPEG_BINARY'] || 'ffmpeg'
-  config.x.ffprobe_binary = ENV['FFPROBE_BINARY'] || 'ffprobe'
+  config.x.ffmpeg_binary = ENV.fetch('FFMPEG_BINARY', 'ffmpeg')
+  config.x.ffprobe_binary = ENV.fetch('FFPROBE_BINARY', 'ffprobe')
 end