Fix: handle null document_metadata in kb_prompt to prevent citation crash (#14651) (#14666)

### What problem does this PR solve?

Fixes #14651.

`kb_prompt()` in `rag/prompts/generator.py` crashes with
`AttributeError: 'NoneType' object has no attribute 'items'` during
agent citation generation when a retrieved chunk carries
`document_metadata: null`.

**Root cause.** The crash happens at `rag/prompts/generator.py:132-133`:

```python
meta = ck.get("document_metadata", {})
for k, v in meta.items():
```

`dict.get(key, default)` only returns the default when the key is
*missing*. When the key is present with an explicit `None` value,
`.get()` returns `None`, and `.items()` crashes.

**How the chunk gets `None`.** It's a round-trip inside RAGFlow itself,
not bad input from retrieval:

1. The agent stores retrieved chunks via `agent/canvas.py:814`, which
routes them through `chunks_format()`.
2. `rag/prompts/generator.py:61` canonicalizes the field with
`chunk.get("document_metadata")` (no default), so chunks without
metadata become `{"document_metadata": None, ...}`.
3. `agent/component/agent_with_tools.py:314` feeds those canonicalized
chunks back into `kb_prompt()` for citation generation, and
`.get("document_metadata", {})` no longer protects us.

**Fix.** One-line change at `rag/prompts/generator.py:132`: use
`ck.get("document_metadata") or {}` so an explicit `None` is also
coerced to `{}`.

The line-61 `None` is intentionally part of the API/UI contract — the
frontend handles it via optional chaining
(`web/src/components/markdown-content/index.tsx:184`,
`web/src/pages/next-search/search-view.tsx:217`) — so the fix belongs at
the consumer, not the producer.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

rag/prompts/generator.py

@@ -129,7 +129,7 @@ def kb_prompt(kbinfos, max_tokens, hash_id=False):
         cnt = "\nID: {}".format(i if not hash_id else hash_str2int(get_value(ck, "id", "chunk_id"), 500))
         cnt += draw_node("Title", get_value(ck, "docnm_kwd", "document_name"))
         cnt += draw_node("URL", ck.get('url', ''))
-        meta = ck.get("document_metadata", {})
+        meta = ck.get("document_metadata") or {}
         for k, v in meta.items():
             cnt += draw_node(k, v)
         cnt += "\n└── Content:\n"

test/unit_test/rag/prompts/test_kb_prompt_metadata.py

@@ -0,0 +1,87 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+import pytest
+
+from rag.prompts.generator import kb_prompt
+
+
+@pytest.mark.p1
+class TestKbPromptDocumentMetadata:
+    """Regression tests for kb_prompt's handling of `document_metadata` on chunks."""
+
+    @pytest.mark.p1
+    def test_null_document_metadata_does_not_crash(self):
+        """A chunk with `document_metadata: None` must not raise AttributeError.
+
+        Regression for issue #14651: chunks retrieved from the index can carry
+        an explicit null metadata field, which made `dict.get(..., {})` return
+        `None` and crash citation generation with
+        `AttributeError: 'NoneType' object has no attribute 'items'`.
+        """
+        kbinfos = {
+            "chunks": [
+                {
+                    "id": "chunk-1",
+                    "content_with_weight": "hello world",
+                    "docnm_kwd": "doc.pdf",
+                    "document_metadata": None,
+                }
+            ]
+        }
+
+        rendered = kb_prompt(kbinfos, max_tokens=10000)
+
+        assert len(rendered) == 1
+        assert "hello world" in rendered[0]
+        assert "doc.pdf" in rendered[0]
+
+    @pytest.mark.p1
+    def test_missing_document_metadata_key(self):
+        """A chunk with no `document_metadata` key at all should also work."""
+        kbinfos = {
+            "chunks": [
+                {
+                    "id": "chunk-1",
+                    "content_with_weight": "hello world",
+                    "docnm_kwd": "doc.pdf",
+                }
+            ]
+        }
+
+        rendered = kb_prompt(kbinfos, max_tokens=10000)
+
+        assert len(rendered) == 1
+        assert "hello world" in rendered[0]
+
+    @pytest.mark.p1
+    def test_populated_document_metadata_renders_fields(self):
+        """When metadata is a dict, its key/value pairs must be rendered."""
+        kbinfos = {
+            "chunks": [
+                {
+                    "id": "chunk-1",
+                    "content_with_weight": "hello world",
+                    "docnm_kwd": "doc.pdf",
+                    "document_metadata": {"author": "alice", "year": "2026"},
+                }
+            ]
+        }
+
+        rendered = kb_prompt(kbinfos, max_tokens=10000)
+
+        assert len(rendered) == 1
+        assert "author: alice" in rendered[0]
+        assert "year: 2026" in rendered[0]