Adding unit tests for element not found in max-heap

Author: aparibocci

data_structures/heaps/max_heap_test.rb

@@ -22,12 +22,22 @@ def test_max_returns_maximum_heap_element
     assert heap.max == 4
   end
 
+  def test_max_returns_nil_if_empty_heap
+    heap = MaxHeap.new
+    assert heap.max.nil?
+  end
+
   def test_extract_max_returns_and_removes_maximum_heap_element
     heap = MaxHeap.new([4, 1, 3])
     assert heap.extract_max == 4
     assert heap.to_array == [3, 1]
   end
 
+  def test_extract_max_returns_nil_if_empty_heap
+    heap = MaxHeap.new
+    assert heap.extract_max.nil?
+  end
+
   def test_insert_adds_element_to_appropriate_position
     heap = MaxHeap.new([4, 1, 3])
     heap.insert(2)