refactor: merge build_null_comparison into build_null_difference

Two methods that always composed together — build_null_comparison
created a frozen Comparison, build_null_difference wrapped it in a
Difference. Merged into single build_null_difference method.

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

Author: pftg

lib/capybara/screenshot/diff/image_compare.rb

@@ -168,12 +168,9 @@ def load_comparison(base_path, new_path, options)
           image_preprocessor.process_comparison(comparison)
         end
 
-        def build_null_difference(failed_by = nil, comparison = nil)
-          Difference.build_null(comparison || build_null_comparison, base_image_path, image_path, failed_by)
-        end
-
-        def build_null_comparison
-          Comparison.new(nil, nil, driver_options, driver, image_path, base_image_path).freeze
+        def build_null_difference(failed_by = nil)
+          comparison = Comparison.new(nil, nil, driver_options, driver, image_path, base_image_path).freeze
+          Difference.build_null(comparison, base_image_path, image_path, failed_by)
         end
 
         # Check if both images exist