[BUG] danger.gitlab.mr.description can be null

[xaverlutz]

Describe the bug
danger.gitlab.mr.description is null if no description was set in the request body when the MR was generated via the GitLab API.

Steps to reproduce

1. Create an MR in GitLab with the API
2. Check for includes in the description

Expected behavior
The description should always be an empty string.

Screenshots

Your Environment

software	version
danger.js	11.3.1
node	v20.11.1
npm	10.2.4
Operating System	macOS 14.3.1 (23D60)

Additional context