Attributes are passed through in Title when using empty heading

[cderv]

Related to #8012 report (solves part of it)

We should consider empty headers with attributes when matching against Pandoc Title regex

Dummy example

---
format: html           
---

## {.empty-header}

Content

## Python slide

```{python}
1 + 1
```

It context of revealjs, this is a problem has empty headers can have a meaning (creating a new section without title)

---
format: revealjs           
---

## {.empty-header}

Content

## Python slide

```{python}
1 + 1
```

[dragonstyle]

This looks good to me. I confirmed that Pandoc does parse ## {.foo} as a heading with attributes/classes as you'd expect so we appear to be handling this incorrectly in its current form.