fix(expression): concat return Empty string instead of Null

[lbartoletti]

Description

Fixes #65808 initializing concat variable.

After:

@agiudiceandrea may I ask you to review/test this one?

AI tool usage

• AI tool(s) (Copilot, Claude, or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated. Failure to be honest might result in banning.

[github-actions]

🍎 MacOS Qt6 builds

Download MacOS Qt6 builds of this PR for testing.
This app is not notarized, run sudo xattr -d com.apple.quarantine /Applications/QGIS*.app to avoid the warning
(Built from commit d7b2381)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
To execute locally, unzip the downloaded zip file and run bin\qgis-bin.exe in the extracted directory.
You might be prompted by Windows Defender click "Run anyway"
(Built from commit d7b2381)

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply d7b23815ffa... test(expression): add concat NULL test against empty string
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 set advice.mergeConflict false"

stdout

Auto-merging src/core/expression/qgsexpressionfunction.cpp
[backport-66618-to-queued_ltr_backports 8c0791dd13b] fix(expression): concat return Empty string instead of Null
 Author: Loïc Bartoletti <loic.bartoletti@oslandia.com>
 Date: Tue Jun 30 09:20:30 2026 +0200
 1 file changed, 1 insertion(+), 1 deletion(-)
Auto-merging tests/src/core/testqgsexpression.cpp
CONFLICT (content): Merge conflict in tests/src/core/testqgsexpression.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-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-66618-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 64d90a571140984356cf3044c3ca6e9108f9ffb0 d7b23815ffa5a20b2869c21df540ccf08d194710
# Push it to GitHub
git push --set-upstream origin backport-66618-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-66618-to-queued_ltr_backports.