mdformat failure when EM SPACE character is present

[nschloe]

Describe the bug

When running mdformat on

```
a
```
 
# A

("'```\\na\\n```\\n\\u2003\\n# A\\n'") I'm getting an error. Might be related to https://github.com/executablebooks/mdformat/issues/351.

Reproduce the bug

See above.

List your environment

No response