[mdx field] don't add empty lines between list items

[stefanprobst]

repro: https://github.com/stefanprobst/issue-keystatic-md-list

keystatic will save unordered lists by always adding an empty line between list items. for example:

- one

- two

- three

when the mdx is rendered, this will actually wrap each list item in a paragraph:

<ul>
<li>
<p>one</p>
</li>
<li>
<p>two</p>
</li>
<li>
<p>three</p>
</li>
</ul>

which looks like this:

---

when i manually edit the mdx file and remove the empty lines, this will render as:

<ul>
<li>one</li>
<li>two</li>
<li>three</li>
</ul>

---

from the remark docs:

there are two kinds of lists in markdown, tight and loose. Lists are tight by default but if there is a blank line between two list items or between two blocks inside an item, that turns the whole list into a loose list. When turning markdown into HTML, paragraphs in tight lists are not wrapped in <p> tags.