Fix dart-lang/markdown#578: list with checkbox mixed with empty lines (dart-lang/markdown#583)

pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart

diff --git a/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart
index c911ee7..d8711bb 100644
--- a/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart
+++ b/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart
@@ -214,6 +214,7 @@
           indent = precedingWhitespaces + contentWhitespances;
         }
 
+        taskListItemState = null;
         var content = contentBlockStart != null && !isBlank
             ? parseTaskListItem(textParser.substring(contentBlockStart))
             : '';

pkgs/markdown/test/extensions/ordered_list_with_checkboxes.unit

diff --git a/pkgs/markdown/test/extensions/ordered_list_with_checkboxes.unit b/pkgs/markdown/test/extensions/ordered_list_with_checkboxes.unit
index 3dce7b7..8503e56 100644
--- a/pkgs/markdown/test/extensions/ordered_list_with_checkboxes.unit
+++ b/pkgs/markdown/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>
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