[processing] save NULL and empty values into different files in Split Vector Layer algorithm (fix #38105)

[alexbruy]

Description

The Split Vector Layer algorithm exports both NULL and empty attribute values to the same filename, as a result one file overwrites another and some results are lost. Proposed PR assigns different suffixes for such files (_EMPTY and _NULL).

Fixes #38105.

Also fix unreported bug with displaying incorrect number of features written to each output file.

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply 9a3465e88e... ad test
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

[backport-46997-to-queued_ltr_backports 9e53168c37] [processing] save NULL and empty values into different files in Split Vector Layer algorithm (fix #38105)
 Author: Alexander Bruy <alexander.bruy@gmail.com>
 Date: Tue Jan 25 19:48:11 2022 +0200
 1 file changed, 14 insertions(+), 1 deletion(-)
Auto-merging tests/src/analysis/testqgsprocessingalgs.cpp
CONFLICT (content): Merge conflict in tests/src/analysis/testqgsprocessingalgs.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-46997-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 0780bf08a0126cbc5adb7125bd06bc59a2010cad,9a3465e88ec884f0b5f6881111f45679b0559487
# Push it to GitHub
git push --set-upstream origin backport-46997-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-46997-to-queued_ltr_backports.

[qgis-bot]

The backport to failed failed:

The process '/usr/bin/git' failed with exit code 128

stderr

fatal: invalid reference: failed

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-failed failed
# Navigate to the new working tree
cd .worktrees/backport-failed
# Create a new branch
git switch --create backport-46997-to-failed
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 0780bf08a0126cbc5adb7125bd06bc59a2010cad,9a3465e88ec884f0b5f6881111f45679b0559487
# Push it to GitHub
git push --set-upstream origin backport-46997-to-failed
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-failed

Then, create a pull request where the base branch is failed and the compare/head branch is backport-46997-to-failed.

[m-kuhn]

FYI: Backport bot updated to use failed backport instead of backport failed to avoid the backport loop issue in the future. CC @3nids