Handle quote having no rendered children

We have a block like

```json
{
  "type": "quote",
  "blockId": “<snip>",
  "parent": "> <u>Table of Contents</u>",
  "children": [
    {
      "type": "table_of_contents",
      "blockId": "<snip>",
      "parent": "",
      "children": []
    }
  ]
}
```

where `mdstr` ends up being `{}`, which without this causes a crash

Author: tjenkinson

src/notion-to-md.ts

@@ -142,7 +142,7 @@ export class NotionToMarkdown {
             nestingLevel,
           );
 
-          const formattedContent = (mdstr.parent ?? mdstr[pageIdentifier])
+          const formattedContent = (mdstr.parent ?? mdstr[pageIdentifier] ?? '')
             .split("\n")
             .map((line) => (line.trim() ? `> ${line}` : ">"))
             .join("\n")