### Summary
Handle searching dataset without embedding model
In this PR, Searching datasets with different embedding models or
searching dataset with/without embedding models are not allowed. We will
improve the behavior later.
## Summary
Fixes#15532 — `delete_datasets()` crashes with `IndexError` when a
document has no `File2Document` row.
`delete_datasets()` in `dataset_api_service.py` called
`File2DocumentService.get_by_document_id()` and immediately accessed
`f2d[0].file_id` without checking whether the lookup returned any rows.
Documents created via API ingestion or connector sync may exist without
a linked file record, causing dataset deletion to abort with HTTP 500.
This PR mirrors the existing guard already used in `file_service.py` and
`document_api_service.py`.
