Fix resetting namespace for environment in UI has no effect

What does this MR do and why?

There's a bug preventing to deselect Kubernetes namespace that was previously selected for an environment.

This MR fixes the bug by explicitly emitting a null value instead of undefined as undefined got ignored by the GraphQl mutation.

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Screenshots or screen recordings

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Screen_Recording_2024-03-04_at_22.14.38

How to set up and validate locally

1. Visit Project -> Operate -> Environments if the new navigation is enabled

2. Create/Edit an environment using the UI

3. Enable KAS on your GDK (steps 1-2 from the guide).

4. Visit the Project -> Infrastructure -> Kubernetes clusters page and create an agent following the instructions from the modal.

   • Select the "Connect a cluster" button
   • The modal should pop up
   • In the modal select "Select an agent or enter a name to create new"
   • You probably won't have any configured agents to show up in the list, create a new one by typing the name of your choice
   • The button should appear at the bottom of the list saying "Create agent: <your-agent-name>"
   • Select the button and click "Register" in the next view.
   • Save the token to use it in the next point.

5. Add the following configuration inside your project in .gitlab/agents/<your-agent-name>/config.yaml for the user_access agent:

   user_access:
     access_as: 
       agent: {}
     projects:
     - id: <your-group>/<your-project-to-share-agent-with>

6. Note that the shared agents should be connected to the cluster in order to appear in the list. Please follow points 3-8 from the guide and then the Deploy the GitLab Agent (agentk) with k3d section to create a local cluster and connect your agent with the cluster.

7. Select an agent from the dropdown in the Environments settings page - the namespace selector will appear.

8. Select a namespace and save the environment.

9. Edit the environment and reset the namespace selection.

10. Save and verify namespace was deselected.

Related to #444312 (closed)