fix: prevent None values in auto-metadata from causing KeyError (#15842)

## Problem

When users configure auto-metadata for a dataset, parsing crashes with:

```
KeyError: 'properties' in gen_metadata → schema["properties"]
```

## Root Cause

Pydantic `AutoMetadataField` defaults `enum` and `description` to `None`
when the frontend omits these fields:

```python
class AutoMetadataField(Base):
    enum: Annotated[list[str] | None, Field(default=None)]
    description: Annotated[str | None, Field(default=None)]
```

These `None` values propagate through the call chain and cause two
crashes:

rag/prompts/generator.py

@@ -955,6 +955,11 @@ async def relevant_chunks_with_toc(query: str, toc: list[dict], chat_mdl, topn:
 
 META_DATA = load_prompt("meta_data")
 async def gen_metadata(chat_mdl, schema: dict, content: str):
+    if not schema:
+        return ""
+    if "properties" not in schema:
+        logging.warning("gen_metadata: schema has no 'properties' key: %s", schema)
+        return ""
     template = PROMPT_JINJA_ENV.from_string(META_DATA)
     for k, desc in schema["properties"].items():
         if "enum" in desc and not desc.get("enum"):