fixing null reference in yarn.lock check (#3444)

lib/linguist/generated.rb

@@ -486,6 +486,7 @@ module Linguist
     # Returns true or false.
     def generated_yarn_lock?
       return false unless name.match(/yarn\.lock/)
+      return false unless lines.count > 0
       return lines[0].include?("# THIS IS AN AUTOGENERATED FILE")
     end
   end