[Server] Extend the WMS FILTER whitelist with IS, NOT, NULL

[rldhont]

The QGIS Server Filter parameter accept OGC Filter and an SQL where clause.

In the case of the SQL where clause, to avoid injection and garanty some safety, QGIS Server uses a whitelist of allowed characters and keywords.
The whitelist was AND,OR,IN,=,<,>=,>,>=,!=,',',(,),DMETAPHONE,SOUNDEX.

We need to be able to handle NULL values.

Description

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[pblottiere]

If memory serves me right, I think that this PR fixes an issue raised by @elemoine some weeks ago, but I didn't succeed in retrieving the dedicated ticket...

[elemoine]

If memory serves me right, I think that this PR fixes an issue raised by @elemoine some weeks ago, but I didn't succeed in retrieving the dedicated ticket...

Indeed.

And the change looks good to me.

[rldhont]

@mhugent, @elpaso any objections to merge?

Would someone like it to backport?

[elpaso]

+1 for merge and backport

[backporting]

The backport to release-3_4 failed:

Commits ["2c2197867c4901c5353cb262b3b54ac155991f7c"] could not be cherry-picked on top of release-3_4

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport release-3_4
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 2c2197867c4901c5353cb262b3b54ac155991f7c
# Create a new branch with these backported commits.
git checkout -b backport-31517-to-release-3_4
# Push it to GitHub.
git push --set-upstream origin backport-31517-to-release-3_4
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is release-3_4 and the compare/head branch is backport-31517-to-release-3_4.