Fix widget wrong values 0 doubles

[elpaso]

Fixes #34118

Because:

QVariant(0.0) == QVariant(QVariant.Double) -> True

but:

QVariant(0.0).isNull() == QVariant(QVariant.Double).isNull() -> False

[troopa81]

looks ok to me, thanks @elpaso

[elpaso]

@troopa81 this needs backporting, correct?

[qgis-bot]

The backport to release-3_10 failed:

The process '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_10 release-3_10
# Navigate to the new working tree
cd .worktrees/backport-release-3_10
# Create a new branch
git switch --create backport-34152-to-release-3_10
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 4ca2bbba94860b55a207446e0650cc4709708e8d
# Push it to GitHub
git push --set-upstream origin backport-34152-to-release-3_10
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_10

Then, create a pull request where the base branch is release-3_10 and the compare/head branch is backport-34152-to-release-3_10.

[troopa81]

@troopa81 this needs backporting, correct?

Yes, I think so