Ensure doctype name is set to null when missing

This change ensures that the tokenizer sets the doctype name to null
when the doctype name is missing in the input source.

Otherwise, without this change, the doctype name is set to the empty
string — which doesn’t conform to the requirements in the HTML spec, and
which causes us to fail 9 tests in the html5lib-tests suite.

Relates to #35

Author: sideshowbarker

src/nu/validator/htmlparser/impl/Tokenizer.java

@@ -6119,7 +6119,7 @@ private void initDoctypeFields() {
         // Discard the characters "DOCTYPE" accumulated as a potential bogus
         // comment into strBuf.
         clearStrBufAfterUse();
-        doctypeName = "";
+        doctypeName = null;
         if (systemIdentifier != null) {
             Portability.releaseString(systemIdentifier);
             systemIdentifier = null;
@@ -6430,7 +6430,7 @@ public void eof() throws SAXException {
                          * Create a new DOCTYPE token. Set its force-quirks flag
                          * to on.
                          */
-                        doctypeName = "";
+                        doctypeName = null;
                         if (systemIdentifier != null) {
                             Portability.releaseString(systemIdentifier);
                             systemIdentifier = null;