Fix calculation of string stats when nulls are present on Qt 6 builds

[nyalldawson]

No description provided.

[nirvn]

A fix in src/core under Qt6? Nice!

[qgis-bot]

The backport to release-3_34 failed:

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

stderr

error: could not apply 42512dc24a9... Fix calculation of string stats when nulls are present on Qt 6 builds
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/core/qgsstringstatisticalsummary.cpp
Auto-merging tests/src/python/test_qgsstringstatisticalsummary.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgsstringstatisticalsummary.py

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-56305-to-release-3_34
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 42512dc24a92a62371c8d1df19c6e909603310b4,980920eed7cdd53f363e08932e6938e1042092e4
# Push it to GitHub
git push --set-upstream origin backport-56305-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-56305-to-release-3_34.