Small readability improvements in Parser

Also add a test for a parser with no data

test/mock_event_handler.h

@@ -1,8 +1,11 @@
 #include "yaml-cpp/emitterstyle.h"
 #include "yaml-cpp/eventhandler.h"
+#include "yaml-cpp/mark.h"
 
 #include "gmock/gmock.h"
 
+#include <string>
+
 namespace YAML {
 
 class MockEventHandler : public EventHandler {

test/parser_test.cpp

@@ -0,0 +1,16 @@
+#include "yaml-cpp/parser.h"
+#include "mock_event_handler.h"
+#include "gtest/gtest.h"
+
+using YAML::Parser;
+using YAML::MockEventHandler;
+using ::testing::StrictMock;
+
+TEST(ParserTest, Empty) {
+    Parser parser;
+
+    EXPECT_FALSE(parser);
+
+    StrictMock<MockEventHandler> handler;
+    EXPECT_FALSE(parser.HandleNextDocument(handler));
+}