Return nil from Iteration#perform

`perform` should be treated as `void`, with consumers not coupling to
its return value.

Explicitly returning `nil` ensures the return value does not change if
we happen to change the implementation.

lib/job-iteration/iteration.rb

@@ -125,6 +125,8 @@ def deserialize(job_data) # @private
 
     def perform(*params) # @private
       interruptible_perform(*params)
+
+      nil
     end
 
     def retry_job(*, **)

test/unit/iteration_test.rb

@@ -485,6 +485,11 @@ def test_nested_each_iteration_instrumentation
     assert_equal(expected, events)
   end
 
+  def test_perform_returns_nil
+    # i.e. perform is "void", and nobody should depend on the return value
+    assert_nil(JobWithRightMethods.perform_now({}))
+  end
+
   private
 
   # Allows building job classes that read max_job_runtime during the test,