Fix transcript flash upon comment submission

[jcmorrow]

Summary

I hit this once in development, it took a very long time to solve, but eventually I tracked it down to a missing field while writing to the Apollo cache. I thought the bug was gone, but then on production it started happening again. I carefully (I thought) compared the objects that I was pulling from and writing to the cache, and didn't spot any immediate differences, but then I found this issue: apollographql/apollo-client#6375. This is the exact issue we are experiencing here, and the fact that someone mentioned null vs. undefined made me wonder if I had missed a small difference. It turns out, that we were writing primaryLabel: undefined, secondaryLabel: undefined into the cache when making comments that did not have labels. The cache was expecting null values when those are missing. It was silently failing, as described in the issue, and then (I think) refetching from the server because it was effectively a cache-miss (I didn't investigate this part too thoroughly since it wasn't super relevant).

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/recordreplay/devtools/GJvA1xxk6wMiqafGWdbesiGkbzUx
✅ Preview: https://devtools-git-jm-fix-flash-comments-recordreplay.vercel.app