Fix reading empty token stack with a node with properties but no scalar.

E.g. `!2`.

src/singledocparser.cpp

@@ -79,6 +79,12 @@ void SingleDocParser::HandleNode(EventHandler& eventHandler) {
   if (!anchor_name.empty())
     eventHandler.OnAnchor(mark, anchor_name);
 
+  // after parsing properties, an empty node is again a possibility
+  if (m_scanner.empty()) {
+    eventHandler.OnNull(mark, anchor);
+    return;
+  }
+
   const Token& token = m_scanner.peek();
 
   if (token.type == Token::PLAIN_SCALAR && IsNullString(token.value)) {

test/integration/load_node_test.cpp

@@ -257,5 +257,10 @@ TEST(NodeTest, LoadTagWithParenthesis) {
     EXPECT_EQ(node.as<std::string>(), "foo");
 }
 
+TEST(NodeTest, LoadTagWithNullScalar) {
+  Node node = Load("!2");
+  EXPECT_TRUE(node.IsNull());
+}
+
 }  // namespace
 }  // namespace YAML