fix(ci): pass emtpy body when creating release PRs (#11203)

## Problem
#11061 changed release pr creation, and I missed that creating PRs using
`gh` in non-interactive environments *requires* `--body` instead of
defaulting to an empty body.

## Summary of changes
Explicitly set an empty body when creating release PRs.

.github/workflows/_create-release-pr.yml

@@ -87,5 +87,6 @@ jobs:
         TITLE: ${{ steps.vars.outputs.title }}
       run: |
         gh pr create --title "${TITLE}" \
+                     --body "" \
                      --head "${RC_BRANCH}" \
                      --base "${RELEASE_BRANCH}"