Fix rake rdoc when the timestamp file is empty

This happens when rdoc fails during generation as the timestamp
is only written on successful attemps. The file is created anyways though

Author: Earlopain

railties/lib/rails/api/task.rb

@@ -160,8 +160,9 @@ def configure_rdoc_files
 
         # Only generate documentation for files that have been
         # changed since the API was generated.
-        if Dir.exist?(api_dir) && !ENV["ALL"]
-          last_generation = DateTime.rfc2822(File.open("#{api_dir}/created.rid", &:readline))
+        timestamp_path = "#{api_dir}/created.rid"
+        if File.exist?(timestamp_path) && !File.zero?(timestamp_path) && !ENV["ALL"]
+          last_generation = DateTime.rfc2822(File.open(timestamp_path, &:readline))
 
           rdoc_files.keep_if do |file|
             File.mtime(file).to_datetime > last_generation