Fix parsing of nested markdown lists without empty line

[Jinderamarak]

This change fixes issue #2640 and aligns the rendering of markdown more closely with other products (github, preview.md, etc.).

Testing markdown:

**Expected**

- first
- second

    - nested first
    - nested second

- third


**Actual**

- first
- second
    - nested first
    - nested second
- third


**Third**

- first
-
    - nested first
    - nested second
- third

Github

Expected

• first

• second

  • nested first
  • nested second

• third

Actual

• first
• second
  • nested first
  • nested second
• third

Third

• first
• • nested first
  • nested second
• third

Iced