Restore original output_directory instead of nil

More robust and future-proof to restore the previous state
rather than destroying it.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dpep

spec/spec_helper.rb

@@ -19,12 +19,13 @@
 
 RSpec.configure do |config|
   config.around do |example|
+    original_output_directory = Singed.output_directory
     Dir.mktmpdir("singed-spec") do |dir|
       Singed.output_directory = dir
       example.run
     end
   ensure
-    Singed.output_directory = nil
+    Singed.output_directory = original_output_directory
   end
   # rspec-expectations config goes here. You can use an alternate
   # assertion/expectation library such as wrong or the stdlib/minitest