Fix a bug when parsing a list with line breaks

[d0819]

Hello👋

I am using tryfabric/mack on Slack and I found a bug that I would like to fix with this PR.

Issue

I have found that the code is not converting lists with line breaks correctly. For example:

1.
Monday
2.
Tuesday
3.
Wednesday

The output of the conversion looks like this:

\n\n

Solution

I have fixed this issue by modifying the parseList() function. The issue was caused by the function returning an empty string when tokens[0] was empty, but the actual content could be in tokens[1].

The tokens array could look like this, with one space and one text:

{
  "type": "list_item",
  "raw": "1.\na\n",
  "task": false,
  "loose": false,
  "text": "\na",
  "tokens": [
    {"type": "space", "raw": "\n"},
    {
      "type": "text",
      "raw": "a",
      "text": "a",
      "tokens": [{"type": "text", "raw": "a", "text": "a"}]
    }
  ]
}