fix missing signal on QgsCheckableComboBox

[3nids]

No description provided.

[qgis-bot]

The backport to release-3_16 failed:

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

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_16 release-3_16
# Navigate to the new working tree
cd .worktrees/backport-release-3_16
# Create a new branch
git switch --create backport-43487-to-release-3_16
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick b8bdd948e241540d6860a9e2c17282d41fa3c2ac
# Push it to GitHub
git push --set-upstream origin backport-43487-to-release-3_16
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_16

Then, create a pull request where the base branch is release-3_16 and the compare/head branch is backport-43487-to-release-3_16.