Extra newlines inside preformatted sections are removed

[makew0rld]

This is due to the hack in https://github.com/makeworld-the-better-one/md2gemini/commit/1bcd5e7834c80104fca3d531af61d6edb1425183

➤ printf '```\ntest\n\n\ntest2\n```\nokay' | md2gemini
```
test

test2
```

okay

➤ printf '```\ntest\n\n\ntest2\n```\nokay'            
```
test


test2
```
okay