Migration of releases fails if release body is null #188

Hi. Thanks for maintaining this awesome tool.

Migration of GitLab releases fails if description field is null. This is because GitLab assumes null for an optional field but node-gitlab-2-github doesn't

Tracing back the cause, as described in the issue above, it was due to the type definition of gitbeaker's Release.description, which node-gitlab-2-github just trusts it. I asked gitbeaker's author to fix it and he has already fixed it. Therefore, the problem will be resolved if node-gitlab-2-github uses gitbeaker 39.7.0, which would (unfortunately) need some other changes.