Avoid crash when using geometry generators and point displacement renderer

[troopa81]

Fixes #51070

There is no point in using geometry generators and point displacement renderer, because the last relies on symbol size to lay out the point on a grid or radius, which is unknown for geometry generator.

symbol size being 0, computed circle radius was -1 and so we switch to grid rendering and crash before the PR.

[qgis-bot]

The backport to release-3_28 failed:

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

stderr

error: could not apply c1993aa403... Fixes #51070 : Avoid crash when using geometry generators and point
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 tests/src/python/test_qgspointdisplacementrenderer.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgspointdisplacementrenderer.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_28 release-3_28
# Navigate to the new working tree
cd .worktrees/backport-release-3_28
# Create a new branch
git switch --create backport-51784-to-release-3_28
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick c1993aa403ccb50f1c038b81f3432cf7fb102a0d
# Push it to GitHub
git push --set-upstream origin backport-51784-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-51784-to-release-3_28.