[processing] don't try to dissolve buffer results if there are no features (fix #46396)

[alexbruy]

Description

Buffering an empty layer with native "Buffer" algorithm produces empty output layer, while the same operation with enabled "Dissolve result" option ends with error message

Feature could not be written to Buffered_a03cb901_262a_4ed5_b22c_23aeb10000b1: Could not add feature with geometry type GeometryCollection to layer of type MultiPolygon
Could not write feature into OUTPUT
Execution failed after 0.01 seconds

This creates problems when algorithm is used as part of a model. For consistence it is better to produce empty layer in both cases.

Fixes #46396.

[nyalldawson]

Could we get a test for this?

[alexbruy]

Test added

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply 913a4fce0e... add test for buffering empty layer with dissolve
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-47348-to-queued_ltr_backports 7d76f784e4] [processing] don't try to dissolve buffer results if there are no features (fix #46396)
 Author: Alexander Bruy <alexander.bruy@gmail.com>
 Date: Mon Feb 14 18:03:49 2022 +0200
 1 file changed, 1 insertion(+), 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-47348-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick d4c148e03d3f5bb93dc0d5a7f883ffa83b5c206b,913a4fce0e9ca2c08557b1e225e25104fa10550f
# Push it to GitHub
git push --set-upstream origin backport-47348-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-47348-to-queued_ltr_backports.

[agiudiceandrea]

@alexbruy please see a similar issue reported at #53360 (new issue at #53392) about the Dissolve processing algorithm.