fix: exit early if the component receives no documents (#7732)

* exit early if the component receives no documents

* relnote

haystack/components/readers/extractive.py

@@ -210,6 +210,7 @@ class ExtractiveReader:
         """
         texts = []
         document_ids = []
+        document_contents = []
         for i, doc in enumerate(documents):
             if doc.content is None:
                 warnings.warn(
@@ -219,9 +220,11 @@ class ExtractiveReader:
                 continue
             texts.append(doc.content)
             document_ids.append(i)
+            document_contents.append(doc.content)
+
         encodings_pt = self.tokenizer(  # type: ignore
             queries,
-            [document.content for document in documents],
+            document_contents,
             padding=True,
             truncation=True,
             max_length=max_seq_length,
@@ -571,6 +574,9 @@ class ExtractiveReader:
         :raises ComponentError:
             If the component was not warmed up by calling 'warm_up()' before.
         """
+        if not documents:
+            return {"answers": []}
+
         queries = [query]  # Temporary solution until we have decided what batching should look like in v2
         nested_documents = [documents]
         if self.model is None:

releasenotes/notes/reader-crash-no-docs-53085ce48baaae81.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Return an empty list of answers when `ExtractiveReader` receives an empty list of documents instead of raising an exception.

test/components/readers/test_extractive.py

@@ -266,13 +266,17 @@ def test_from_dict_no_token():
     assert component.token is None
 
 
+def test_run_no_docs(mock_reader: ExtractiveReader):
+    mock_reader.warm_up()
+    assert mock_reader.run(query="hello", documents=[]) == {"answers": []}
+
+
 def test_output(mock_reader: ExtractiveReader):
-    answers = mock_reader.run(example_queries[0], example_documents[0], top_k=3)[
-        "answers"
-    ]  # [0] Uncomment and remove first two indices when batching support is reintroduced
+    answers = mock_reader.run(example_queries[0], example_documents[0], top_k=3)["answers"]
     doc_ids = set()
     no_answer_prob = 1
     for doc, answer in zip(example_documents[0], answers[:3]):
+        assert answer.document_offset is not None
         assert answer.document_offset.start == 11
         assert answer.document_offset.end == 16
         assert doc.content is not None