Fix crash in merge features dialog when a field has a unique constraint set

[nyalldawson]

Fixes #54856

[qgis-bot]

The backport to release-3_28 failed:

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

stderr

error: could not apply ec631e820d... Fix crash in merge features dialog when a field has a unique
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".

stdout

Auto-merging src/app/qgsmergeattributesdialog.cpp
CONFLICT (content): Merge conflict in src/app/qgsmergeattributesdialog.cpp
Auto-merging tests/src/app/testqgsmergeattributesdialog.cpp
CONFLICT (content): Merge conflict in tests/src/app/testqgsmergeattributesdialog.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_28 release-3_28
# Navigate to the new working tree
cd .worktrees/backport-release-3_28
# Create a new branch
git switch --create backport-54862-to-release-3_28
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick ec631e820d9ceed848c44e0e2f11d902eafcb42d
# Push it to GitHub
git push --set-upstream origin backport-54862-to-release-3_28
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_28

Then, create a pull request where the base branch is release-3_28 and the compare/head branch is backport-54862-to-release-3_28.