must_include check on description fails with UNKNOWN_INPUT_TYPE_ERROR if description is empty

[link2xt]

Example of config is here: https://github.com/deltachat/deltachat-core-rust/blob/5f779ca9b2fd8f798feac880438e6a840fa35798/.github/mergeable.yml

Even when changelog is not required or provided, mergeable still fails in must_include check with "Input type invalid, expected either string or array of string as input", for example here: https://github.com/deltachat/deltachat-core-rust/pull/3505