Fix coverage of Mutant::AST namespace

spec/unit/mutant/ast/named_children_spec.rb

@@ -42,10 +42,20 @@ RSpec.describe Mutant::AST::NamedChildren do
       end
 
       describe '#bar' do
+        context 'when node is present' do
           it 'returns named child bar' do
             expect(instance.bar).to be(node_bar)
           end
         end
+
+        context 'when node is NOT present' do
+          let(:node) { s(:node, node_foo) }
+
+          it 'returns nil' do
+            expect(instance.bar).to be(nil)
+          end
+        end
+      end
     end
   end
 end