Event Lineage: Create Transform Function for Not Filter

**Problem**
Transform functions are how we track changes to eventtypes as events flow through the system. To support the not filter, we should implement a transform function to represent how a given eventtype becomes "narrower" as it passes the filter. Specifically, this transform function should:
1. Check if the currently eventtype passes the filter
2. If it does, return `nil`

**[Persona:](https://github.com/knative/eventing/blob/main/docs/personas.md)**
Which persona is this feature for?

**Exit Criteria**
A measurable (binary) test that would indicate that the problem has been resolved.

**Time Estimate (optional):**
How many developer-days do you think this may take to resolve?

**Additional context (optional)**
Add any other context about the feature request here.

No file or test is named in the issue. Locate the existing transform-function implementations and the not-filter entry point, then trace how eventtypes are evaluated. Done means a focused test verifies that an eventtype passing the not filter produces a nil transform result.

Written by the indexing model from the issue text.