`attributes` commonmark extension swallows attributes

**Explain the problem.**
```
$ printf '{x=3}' | pandoc -f commonmark+attributes -t native
[]
$ printf 'hello\n\n{x=3}' | pandoc -f commonmark+attributes

hello

If there is nothing the attribute belongs to it is simply swallowed. I find this problematic since this way you are very unlikely to spot such mistakes. I would prefer the markup `{x=3}` to show up as plain text in the output so that you can spot the error while looking at the rendered output.

**Pandoc version?**
pandoc 2.16.2 on Linux