fix(mr review): don't query position for webview discussions
This limits the blast radius of the existing GitLab issue gitlab#298827 (closed).
For certain MRs (seems to be a subset of MRs that are coming from forks of the project), asking for discussions with positions fails. The GraphQL endpoint returns 500.
gitlab#298827 (closed) is going to affect the MR review diff comments (#266 (closed)) no matter what but this MR limits the blast radius and makes sure that showing comments in the webview
(where the position is not needed) still works.
Related to #266 (closed)