fix: metadata_condition returning all docs when filter matches nothing (#14967)

### What problem does this PR solve?

When _parse_doc_id_filter_with_metadata returns [], the empty list is
falsy so the WHERE id IN (...) clause was silently skipped, causing the
full dataset to be returned instead of an empty result.

Change `if doc_ids:` to `if doc_ids is not None:` in both get_list() and
get_by_kb_id() to distinguish between no filter (None) and a filter that
matched zero documents ([]).

Fixes #14962

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/restful_apis/document_api.py

@@ -995,7 +995,7 @@ def _parse_doc_id_filter_with_metadata(req, kb_id):
             if not doc_ids_filter:
                 return RetCode.SUCCESS, "", [], return_empty_metadata
 
-    return RetCode.SUCCESS, "", list(doc_ids_filter) if doc_ids_filter is not None else [], return_empty_metadata
+    return RetCode.SUCCESS, "", list(doc_ids_filter) if doc_ids_filter is not None else None, return_empty_metadata
 
 
 @manager.route("/datasets/<dataset_id>/documents", methods=["DELETE"])  # noqa: F821

api/db/services/document_service.py

@@ -88,7 +88,7 @@ class DocumentService(CommonService):
             docs = docs.where(cls.model.name == name)
         if keywords:
             docs = docs.where(fn.LOWER(cls.model.name).contains(keywords.lower()))
-        if doc_ids:
+        if doc_ids is not None:
             docs = docs.where(cls.model.id.in_(doc_ids))
         if suffix:
             docs = docs.where(cls.model.suffix.in_(suffix))
@@ -143,7 +143,7 @@ class DocumentService(CommonService):
                 .join(User, on=(cls.model.created_by == User.id), join_type=JOIN.LEFT_OUTER)
                 .where(cls.model.kb_id == kb_id)
             )
-        if doc_ids:
+        if doc_ids is not None:
             docs = docs.where(cls.model.id.in_(doc_ids))
         if run_status:
             docs = docs.where(cls.model.run.in_(run_status))