fallback to {} for None metadata from Chroma (#1714)

The basic vector store example started breaking because `Document`
required `not None` for metadata, but Chroma stores metadata as `None`
if none is provided. This creates a fallback which fixes the basic
tutorial
https://langchain.readthedocs.io/en/latest/modules/indexes/examples/vectorstores.html

Here is the error that was generated

```
Running Chroma using direct local API.
Using DuckDB in-memory for database. Data will be transient.
Traceback (most recent call last):
  File "/Users/jeff/src/temp/langchainchroma/test.py", line 17, in <module>
    docs = docsearch.similarity_search(query)
  File "/Users/jeff/src/langchain/langchain/vectorstores/chroma.py", line 133, in similarity_search
    docs_and_scores = self.similarity_search_with_score(query, k)
  File "/Users/jeff/src/langchain/langchain/vectorstores/chroma.py", line 182, in similarity_search_with_score
    return _results_to_docs_and_scores(results)
  File "/Users/jeff/src/langchain/langchain/vectorstores/chroma.py", line 24, in _results_to_docs_and_scores
    return [
  File "/Users/jeff/src/langchain/langchain/vectorstores/chroma.py", line 27, in <listcomp>
    (Document(page_content=result[0], metadata=result[1]), result[2])
  File "pydantic/main.py", line 331, in pydantic.main.BaseModel.__init__
pydantic.error_wrappers.ValidationError: 1 validation error for Document
metadata
  none is not an allowed value (type=type_error.none.not_allowed)
Exiting: Cleaning up .chroma directory
```

langchain/vectorstores/chroma.py

@@ -24,7 +24,7 @@ def _results_to_docs_and_scores(results: Any) -> List[Tuple[Document, float]]:
     return [
         # TODO: Chroma can do batch querying,
         # we shouldn't hard code to the 1st result
-        (Document(page_content=result[0], metadata=result[1]), result[2])
+        (Document(page_content=result[0], metadata=result[1] or {}), result[2])
         for result in zip(
             results["documents"][0],
             results["metadatas"][0],