fix: avoid triggering `Filter.__bool__` warning in `_equals_none_to_not_exists` (#533)

# Description

**Reviewer**: All code inside `examples/` is generated.

The `Filter` class raises a `UserWarning` when evaluated in a boolean
context to prevent users from incorrectly combining filters with
`and`/`or`.

However, pygen's internal code was triggering this warning by using `if
(res := ...)` instead of `if (res := ...) is not None` when processing
`And`, `Or`, and `Not` filters.

This follows the same fix applied to the SDK in
https://github.com/cognitedata/cognite-sdk-python/pull/2341

## Bump

- [x] Patch
- [ ] Minor
- [ ] Skip

## Changelog
### Fixed

- Use explicit `is not None` checks in `_equals_none_to_not_exists` to
avoid triggering `Filter.__bool__` UserWarning from cognite-sdk-python
