Fix crash on empty polygon conversion to curve

[strk]

Closes GH-50466

Empty polygons have no exterior ring, do not assume they always do

[qgis-bot]

The backport to release-3_22 failed:

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

stderr

error: could not apply 5281ac1045... Fix crash on empty polygon conversion to curve
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/core/geometry/testqgspolygon.cpp
CONFLICT (content): Merge conflict in tests/src/core/geometry/testqgspolygon.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-release-3_22 release-3_22
# Navigate to the new working tree
cd .worktrees/backport-release-3_22
# Create a new branch
git switch --create backport-50577-to-release-3_22
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 5281ac10455d9bb2cfd6876f5063956480a83343
# Push it to GitHub
git push --set-upstream origin backport-50577-to-release-3_22
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_22

Then, create a pull request where the base branch is release-3_22 and the compare/head branch is backport-50577-to-release-3_22.