Handle empty result gracefully in gem pristine --only-missing-extensions

When no gems have missing extensions (e.g. on JRuby where
missing_extensions? always returns false), display an informational
message instead of raising Gem::Exception.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

lib/rubygems/commands/pristine_command.rb

@@ -132,6 +132,11 @@ def execute
     specs = specs.select {|spec| spec.platform == RUBY_ENGINE || Gem::Platform.local === spec.platform || spec.platform == Gem::Platform::RUBY }
 
     if specs.to_a.empty?
+      if options[:only_missing_extensions]
+        say "No gems with missing extensions to restore"
+        return
+      end
+
       raise Gem::Exception,
             "Failed to find gems #{options[:args]} #{options[:version]}"
     end

test/rubygems/test_gem_commands_pristine_command.rb

@@ -248,7 +248,13 @@ def test_execute_extensions_only_missing_extensions
     end
 
     refute_includes @ui.output, "Restored #{a.full_name}"
-    assert_includes @ui.output, "Restored #{b.full_name}"
+
+    if Gem.java_platform?
+      refute_includes @ui.output, "Restored #{b.full_name}"
+      assert_includes @ui.output, "No gems with missing extensions to restore"
+    else
+      assert_includes @ui.output, "Restored #{b.full_name}"
+    end
   end
 
   def test_execute_no_extension