Added test illustrating filtering of null segments

refs https://github.com/TryGhost/Team/issues/861

- The test case illustrates that it's already possible to achieve what's described in the referenced issue (bug where the content is not filtered correctly when a single email segment card is present)
- The bug fix for a general issue should be using this parser's ability to properly filter html/plaintext content

test/unit/services/mega/post-email-serializer_spec.js

@@ -68,5 +68,17 @@ describe('Post Email Serializer', function () {
             output.html.should.eql('hello<div> paid users!</div>');
             output.plaintext.should.eql('hello paid users!');
         });
+
+        it('should hide all segments when the segment filter is empty', function () {
+            const email = {
+                otherProperty: true,
+                html: 'hello<div data-gh-segment="status:free"> free users!</div><div data-gh-segment="status:-free"> paid users!</div>',
+                plaintext: 'test'
+            };
+
+            let output = renderEmailForSegment(email, null);
+            output.html.should.equal('hello');
+            output.plaintext.should.equal('hello');
+        });
     });
 });