fix: PromptNode run raises on empty inputs (#7734)

* fix: PromptNode run raises on empty inputs

* add reno

* convert test into unit test

haystack/nodes/prompt/prompt_node.py

@@ -252,19 +252,19 @@ def _prepare(  # type: ignore
         else:
             invocation_context = invocation_context.copy()
 
-        if query and "query" not in invocation_context:
+        if query is not None and "query" not in invocation_context:
             invocation_context["query"] = query
 
-        if file_paths and "file_paths" not in invocation_context:
+        if file_paths is not None and "file_paths" not in invocation_context:
             invocation_context["file_paths"] = file_paths
 
-        if labels and "labels" not in invocation_context:
+        if labels is not None and "labels" not in invocation_context:
             invocation_context["labels"] = labels
 
-        if documents and "documents" not in invocation_context:
+        if documents is not None and "documents" not in invocation_context:
             invocation_context["documents"] = documents
 
-        if meta and "meta" not in invocation_context:
+        if meta is not None and "meta" not in invocation_context:
             invocation_context["meta"] = meta
 
         if "prompt_template" not in invocation_context:

releasenotes/notes/fix-promptnode-empty-inputs-c050c2040d489f9e.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    When passing empty inputs (such as `query=""`) to PromptNode, the node would raise an error. This has been fixed.

test/prompt/test_prompt_node.py

@@ -1212,3 +1212,19 @@ def test_prompt_no_truncation(mock_model, caplog):
     with caplog.at_level(logging.DEBUG):
         _ = node.prompt(PromptTemplate(prompt))
         assert prompt in caplog.text
+
+
+@pytest.mark.unit
+def test_run_with_empty_inputs():
+    mock_model = MagicMock(spec=PromptModel)
+    mock_model.invoke.return_value = ["mock answer"]
+    node = PromptNode(mock_model, default_prompt_template="question-answering")
+    result, _ = node.run(query="", documents=[])
+
+    # validate output variable present
+    assert "answers" in result
+    assert len(result["answers"]) == 1
+
+    # and that so-called invocation context contains the right keys
+    assert "invocation_context" in result
+    assert all(item in result["invocation_context"] for item in ["query", "documents", "answers", "prompts"])