Layout specified in site.json is not applied when page does not have front matter

[Xenonym]

Tell us about your environment

• MarkBind Version: 1.17.0

What did you do? Please include the actual source code causing the issue.
I created a MarkBind site with two layouts, default and testLayout.

In _markbind/layouts/default/footer.md:

<footer>
  <div class="text-center">
    Default footer
  </div>
</footer>

In _markbind/layouts/testLayout/footer.md:

<footer>
  <div class="text-center">
    testLayout footer
  </div>
</footer>

I then specified that "layout: "testLayout" for index.md in site.json:

{
  "src": "index.md",
  "title": "Hello World",
  "layout": "testLayout"
},

In index.md:

# Hello world
Welcome to your page generated with MarkBind.

I then did markbind serve.

What did you expect to happen?
I expected testLayout to apply to index.md, displaying testLayout footer for the footer.

What actually happened? Please include the actual, raw output.
The footer from the default layout displayed instead. This also happens when a empty <frontmatter> is included.

If I include a <frontmatter> with content in index.md, then testLayout does get applied:

<frontmatter>
	title: Hello World
</frontmatter>
# Hello world
Welcome to your page generated with MarkBind.

[ang-zeyu]

oops. fixed in #1321