Fix #578: list with checkbox mixed with empty lines

lib/src/block_syntaxes/list_syntax.dart

@@ -214,6 +214,7 @@ abstract class ListSyntax extends BlockSyntax {
           indent = precedingWhitespaces + contentWhitespances;
         }
 
+        taskListItemState = null;
         var content = contentBlockStart != null && !isBlank
             ? parseTaskListItem(textParser.substring(contentBlockStart))
             : '';

test/extensions/ordered_list_with_checkboxes.unit

@@ -60,4 +60,20 @@
 <pre><code>[ ] four
 </code></pre>
 </li>
+</ol>
+>>> checkbox with empty content
+1. [ ] one
+2.
+3. 
+4. four
+5. [ ] five
+6.
+<<<
+<ol class="contains-task-list">
+<li class="task-list-item"><input type="checkbox"></input>one</li>
+<li></li>
+<li></li>
+<li>four</li>
+<li class="task-list-item"><input type="checkbox"></input>five</li>
+<li></li>
 </ol>