gitlab-merge error message unclear when no merge request exists

I just stumbled on this because I forgot to open a merge request and tried to use gitlab-merge to merge two branches.

If there is no merge request for the branch being merged, gitlab-merge fails with an unhelpful message:

got unexpected API response: []

The culprit can be found in this statement that should probably do different things if length == 0 (produce a message about no merge requests existing) and type != "array" (produce the unexpected API response error message).