Blank lines in backtick code block on blockquote are removed

[seaoak]

Check List

Please check followings before submitting a new issue.

• I have already read Docs page & Troubleshooting page
• I have already searched existing issues and they are not help to me
• I examined error or warning messages and it's difficult to solve
• Node.js is higher than 6.9

Expected behavior

Blank lines in backtick code block on blockquote should be kept.
But backtick_code_block.js removes them.

For example,

> This is a code-block
> 
> > ```
> >
> >
> >
> {
>   "test": 123
>
> }
>
> ```
> 
> This is the last line

should be rendered to:

<blockquote>
<p>This is a code-block</p>
<blockquote>
<pre><code>



{
  &quot;test&quot;: 123

}

</code></pre>
</blockquote>
<p>This is the last line</p>
</blockquote>

Actual behavior

Blank lines are removed:

<blockquote>
<p>This is a code-block</p>
<blockquote>
<pre><code>
{
  &quot;test&quot;: 123
}
</code></pre>
</blockquote>
<p>This is the last line</p>
</blockquote>

Environment & Settings

• Node.js v8.16.2 (npm v6.4.1)
• Node.js v10.16.3 (npm v6.9.0)
• Node.js v12.11.1 (npm v6.11.3)

Others

This bug is introduced by the PR #3765

I already have a patch for this issue.
I'll make a PR soon.