Describe the bug

The following markdown is a valid footnote:

[^lorem]

[^lorem]:
    ipsum

GitHub screenshot:

However, mdit parses it as if it was invalid becase the first line is empty.

(See executablebooks/mdformat-footnote#6).

Reproduce the bug

See above.

List your environment

• No jupyter.

• python --version
  

  Python 3.10.5
  

• Ubuntu 22.10