Keep skipped jobserver examples from clobbering global state

The before hook skips before it saves the previous connection pool client
and UI, but the after hook restored those unconditionally, leaving
Gem::Request::ConnectionPools.client as nil for every later example. Only
restore what was captured, matching the guard Artifice.deactivate uses.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

spec/bundler/installer/parallel_installer_spec.rb

@@ -132,9 +132,12 @@
       Bundler.ui = Bundler::UI::Silent.new
     end
 
+    # The `before` hook can `skip` before it saves anything, so only restore
+    # what was actually captured. Otherwise every skipped example clobbers the
+    # globals with nil.
     after do
-      Bundler.ui = @old_ui
-      Gem::Request::ConnectionPools.client = @previous_client
+      Bundler.ui = @old_ui if @old_ui
+      Gem::Request::ConnectionPools.client = @previous_client if @previous_client
       Artifice.deactivate
     end