Add defense against null ids

## Problem
We found that new entities being created were causing this module to error, as they do not have ids.

## Environment
- Drupal: [e.g. 10.4.x]
- PHP: [e.g. 8.3]
- migrate_forward_draft: [version / branch]
- Content moderation: [yes/no; workflow name if yes]

## Migration
- Destination plugin: [e.g. entity_with_forward_draft:node]
- migration_sync: [true/false]
- forward_revision_overwrite_properties: [list or “empty”]
- Other notable destination keys: [overwrite_properties, default_bundle, …]

## Reproduction
1. [Step 1 — entity state before migrate, e.g. default vs forward draft]
2. [Step 2 — migration run / drush command]
3. [Step 3 — observed result]

## Expected
Null destination IDs should be treated as a row creation (skip carry-forward logic)

## Actual
Error attempting to treat the row as an update (carry-forward logic on non-existing row)

## Attachments
- [ ] Reduced migration YAML snippet (no secrets)
- [ ] Screenshots or Drush/log excerpts if useful