Fix nested list style in Markdown preview

[hsfzxjy]

fix #190860

There could be two cases for nested list:

- a
  - b
- c

renders into

<ul>
 <li>a
   <ul><li>b</li></ul>
 </li>
 <li>c</li>
</ul>

---

- a
  - b

  some text
- c

renders into

<ul>
 <li><p>a</p>
   <ul><li>b</li></ul>
   <p>some text</p>
 </li>
 <li><p>c</p></li>
</ul>

---

PR #124445 fixed the first case, but did not take the second case into consideration. This PR carries the following modification:

1. Instead of setting all ul ul with margin-bottom: 0, we set only ul ul:first-child, which should match exactly the first case;
2. For the second case, we keep the margin-bottom of nested ul/ol, and further, we adjust li p's margin-bottom to 0.7em, which is the same as that of ul/ol and makes the style more consistent.