#20139: Bug: For Non nullable fields (SELECT, MULTISELECT), do not allow workflow update / create / upsert to set "no selected"

Scope & Context

We have multiple opportunity views to handle different types of situations. Here is a simple example to understand:

• Default views (organizing opportunities by stage) are used to handle all incoming requests.
• When someone puts an opportunity under the "WIN" stage, we add a custom field called winstage set to "New Client".
• There is a view that displays all opportunities with a non-null winstage field, so the marketing team can handle the onboarding in a dedicated view.

Current behavior

Once we set the winstage, the opportunity stays in the WIN stage column in the default view, resulting in an infinite stack of opportunities.

We can't manually remove that stage (set it to null). When using a Workflow to do it and selecting the default "no stage selected," it still doesn't remove the "WIN" value.

The workaround is to create a "HIDDEN" stage value and hide its column in the view.

Expected behavior

It should be possible to have a null stage to hide opportunities in kanban views based on enum fields.