Test #maximum and #minimum with empty enumerable

Follow-up to #41404.

These tests will prevent regressions if we decide to change the
implementations of `maximum` or `minimum` in the future (for example,
calling `max_by` or `min_by` followed by `send`).

activesupport/test/core_ext/enumerable_test.rb

@@ -34,11 +34,21 @@ def test_minimum
     assert_equal 5, payments.minimum(:price)
   end
 
+  def test_minimum_with_empty_enumerable
+    payments = GenericEnumerable.new([])
+    assert_nil payments.minimum(:price)
+  end
+
   def test_maximum
     payments = GenericEnumerable.new([ Payment.new(5), Payment.new(15), Payment.new(10) ])
     assert_equal 15, payments.maximum(:price)
   end
 
+  def test_maximum_with_empty_enumerable
+    payments = GenericEnumerable.new([])
+    assert_nil payments.maximum(:price)
+  end
+
   def test_sums
     enum = GenericEnumerable.new([5, 15, 10])
     assert_equal 30, enum.sum