fix qgsinterpolator with NULL value

[vcloarec]

fix #43286 and #42689

[alexbruy]

@vcloarec maybe it makes sense to add a test to prevent breakage in future?

[nicogodet]

Does it fix #42689 too ?

[vcloarec]

Does it fix #42689 too ?

surely

[vcloarec]

@alexbruy , yes, with tests, it would be better... on my side, no time to write tests for now...

[nyalldawson]

Let's leave this open for now -- I'll add a test then we can merge

[vcloarec]

or @nyalldawson , let me write a test later, I think I could find few time for it before 3.20 release...

[vcloarec]

tests added

[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-43302-to-release-3_16
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 23ffd06cfdad8747f3f7485820bb4ffdfb12fe23,6f736d9bd56023aef43c975dbe15bd3aaa1fee37,e65d3676faff49cb82979f177e7c6c4715997dec
# Push it to GitHub
git push --set-upstream origin backport-43302-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-43302-to-release-3_16.