Add specification for null matcher

spec/unit/mutant/matcher/chain_spec.rb

@@ -27,7 +27,6 @@ describe Mutant::Matcher::Chain do
       end
     end
 
-
     it 'should yield subjects' do
       expect { subject }.to change { yields }.from([]).to([subject_a, subject_b])
     end

spec/unit/mutant/matcher/null_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+
+describe Mutant::Matcher::Null do
+  let(:object) { described_class.new }
+
+  describe '#each' do
+    let(:yields) { [] }
+
+    subject { object.each { |entry| yields << entry } }
+
+    # it_should_behave_like 'an #each method'
+    context 'with no block' do
+      subject { object.each }
+
+      it { should be_instance_of(to_enum.class) }
+
+      it 'yields the expected values' do
+        expect(subject.to_a).to eql(object.to_a)
+      end
+    end
+
+    it 'should yield subjects' do
+      expect { subject }.not_to change { yields }.from([])
+    end
+  end
+end