fix #21931 Mesh calculator crash when used with face dataset

[PeterPetrik]

https://issues.qgis.org/issues/21931

[backporting]

The backport to release-3_4 failed:

Commits ["7c8075825a8b53a5c2727f228420674312c47e24"] could not be cherry-picked on top of release-3_4

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport release-3_4
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 7c8075825a8b53a5c2727f228420674312c47e24
# Create a new branch with these backported commits.
git checkout -b backport-9875-to-release-3_4
# Push it to GitHub.
git push --set-upstream origin backport-9875-to-release-3_4
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is release-3_4 and the compare/head branch is backport-9875-to-release-3_4.