[Postgres] Fix save of cleared attributes

[troopa81]

Fixes #39146 when postgres provider try to save a value coming from default value clause (when we push the clear button in the attribute table for instance)

[qgis-bot]

The backport to queued_ltr_backports failed:

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

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-45176-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick aa4541f50f73ac2c18f80ec255421e2b7bac9860
# Push it to GitHub
git push --set-upstream origin backport-45176-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-45176-to-queued_ltr_backports.