JsonToMd().page2md fails when block["children"] is None

[kapicic]

Description

When calling JsonToMd().page2md on a list of block where at least one block is missing the children key, the conversion fails.

Steps to reproduce:

from notion_client.client import Client as NotionClient
from notion2markdown.json2md import JsonToMd

client = NotionClient(auth=...)
blocks = client.blocks.children.list(page_id)
page_content = JsonToMd().page2md(blocks) # fails KeyError("children")

[kapicic]

The fix is easy, instead of doing value["children"] inside of JsonToMd, just do value.get("children", [])