Set the variable to false instead of checking if it is defined

We already have an initalize method, so we can just set the variable to
false there.

activesupport/lib/active_support/testing/parallelize_executor.rb

@@ -9,6 +9,7 @@ def initialize(size:, with:, threshold: ActiveSupport.test_parallelization_thres
         @size = size
         @parallelize_with = with
         @threshold = threshold
+        @parallelized = false
       end
 
       def start
@@ -49,7 +50,7 @@ def parallelize
         end
 
         def parallelized?
-          @parallelized if defined?(@parallelized)
+          @parallelized
         end
 
         def should_parallelize?