Fix crash when exporting vectors if field domain cannot be retrieved

[nyalldawson]

No description provided.

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 590873f)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 590873f)

[qgis-bot]

The backport to release-3_34 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 590873f55b7... Fix crash when exporting vectors if field domain cannot be retrieved
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging src/core/qgsvectorfilewriter.cpp
CONFLICT (content): Merge conflict in src/core/qgsvectorfilewriter.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_34 release-3_34
# Navigate to the new working tree
cd .worktrees/backport-release-3_34
# Create a new branch
git switch --create backport-59280-to-release-3_34
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 590873f55b7c8e3b47914b3bc62425424d6392af
# Push it to GitHub
git push --set-upstream origin backport-59280-to-release-3_34
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_34

Then, create a pull request where the base branch is release-3_34 and the compare/head branch is backport-59280-to-release-3_34.

[nyalldawson]

3.34 backport is NOT required