Add is not null to query builder

[imilinovic]

Description

Add IS NOT to query builder.

Pull request type

Please delete options that are not relevant.

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring with functional or API changes
• Refactoring without functional or API changes
• Build or packaging related changes
• Documentation content changes
• Other (please describe):

Related issues

closes #293

Delete section if this PR doesn't resolve any issues.

Closes (link to issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

######################################

Reviewer checklist (the reviewer checks this part)

• Core feature implementation
• Tests
• Code documentation
• Documentation on gqlalchemy/docs

######################################

[katarinasupe]

@imilinovic We no longer use the develop branch for the release; everything goes directly to the main.
Why are there 96 changes? It's hard to review.

FYI, as discussed, this will be included in the next release (July 10th). Work on Memgraph is a priority, so we can move the milestone due date if you can't make it.

[antepusic]

LGTM

[katarinasupe]

All looks fine. As soon as deda fixes the workflow, the tests should pass as well.

[katarinasupe]

@imilinovic merge the latest main for tests to pass.
Once the tests pass, feel free to squash and merge.

[katarinasupe]

I extended the description in #293 which better explains what needs to be done. I am closing this PR since it is not fixing that issue.