Block attributes formatting introduce extra empty line

[kdeldycke]

Describe the bug

context When block attributes are formatted, an extra line is added.

expectation I expect the following MyST code to be left as-is by mdformat, as it looks pretty well formatted to me:

{caption="`GROUPS` - Description."}
```mermaid
flowchart
    (...)

**bug**
But instead, the code above gets re-formatted like this:

{caption="GROUPS - Description."}

flowchart
    (...)

Notice the extra blank line introduced.

**problem**
I think this is a problem if I compare all [canonical reference from the MyST-parser documentation](https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#block-attributes).

### Reproduce the bug

````shell-session
$ cat ./block-attributes.md
{caption="`GROUPS` - Description."}
```mermaid
flowchart
    (...)

```shell-session
$ mdformat ./block-attributes.md

$ cat ./block-attributes.md
{caption="`GROUPS` - Description."}

```mermaid
flowchart
    (...)

### List your environment

```shell-session
$ mdformat --version
mdformat 0.7.16 (mdformat_myst: 0.1.5, mdformat_tables: 0.4.1, mdformat_frontmatter: 2.0.1)

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