Simplify test for case without doctype.

No need to test the DOM implementation itself.

test/index.js

@@ -13,10 +13,8 @@ function makeStubDocument(doctype) {
     return doc
 }
 
-test('should return empty string if absent', t => {
-    const doctype = ''
-    const doc = makeStubDocument(doctype)
-    t.is(doctypeToString(doc.doctype), doctype)
+test('should return empty string if doctype is null', t => {
+    t.is(doctypeToString(null), '')
 })
 
 test('should work for a HTML5-ish doctype', t => {