[Bug]: Panic occurred when using import defer

### System Info

@rspack/core 1.6.0

### Details

Panic occurred when using import defer:

<img width="1068" height="176" alt="Image" src="https://github.com/user-attachments/assets/9fc1646b-5e23-46cf-a93f-c51956e85a90" />

### Reproduce link

https://github.com/web-infra-dev/rsbuild/pull/6490

### Reproduce Steps

1. git checkout import_defer_1103
2. pnpm i
3. cd packages/core
4. pnpm build