Fix crash on invalid grass layer paths

[nyalldawson]

Fixes #32130

[lbartoletti]

LGTM

[qgis-bot]

The backport to release-3_28 failed:

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

stderr

error: could not apply f560962110... Fix grass debug output always going to error
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/providers/grass/qgsgrass.cpp
CONFLICT (content): Merge conflict in src/providers/grass/qgsgrass.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_28 release-3_28
# Navigate to the new working tree
cd .worktrees/backport-release-3_28
# Create a new branch
git switch --create backport-53317-to-release-3_28
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick f560962110873222f4713ad5336bd8404c025282,e40cce964b15ae8fab2817147027a7eb90a8a3a2,5cf50e0d3cfaea651c72d3736f3c925008f36de6
# Push it to GitHub
git push --set-upstream origin backport-53317-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-53317-to-release-3_28.