[bug] empty markdown lines cause incorrect comment syntax in q when writing percent notebook

In q, a line with / as the content starts a block comment. Jupytext produces these when converting to percent format and there's an empty line in a markdown cell.

{
 "cells": [
  {
   "cell_type": "markdown",
   "id": "c261d244",
   "metadata": {},
   "source": [
    "This is a markdown cell\n",
    "\n",
    "With a blank line ^"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "ba870045",
   "metadata": {},
   "outputs": [],
   "source": [
    "\n",
    "show \"this is some code now\";"
   ]
  }
 ],
 "metadata": {},
 "nbformat": 4,
 "nbformat_minor": 5
}

jupytext --to q:percent -o - that.ipynb yeilds:

/ %% [markdown]
/ This is a markdown cell
/
/ With a blank line ^

/ %%

show "this is some code now";

The code is commented out though due to the opening block comment on line 3.

The easiest solution (though ugly) is to add a single whitespace after the /. Alternatively, we could change the comment string to //, but this is backwards incompatible.

Another potential solution would be to implement the suggestion from #1627. (Even though that issue was written by an LLM lol), implementing and enabling it by default for q I think would be sufficient to fix my problem.

Anyway, I'll create an MR with my proposed fix, and we can discuss.