chroma[patch]: fix bug (#28538)
Fix bug introduced in
https://github.com/langchain-ai/langchain/pull/27995
If all document IDs are `""`, the chroma SDK will raise
```
DuplicateIDError: Expected IDs to be unique
```
Caught by [docs
tests](https://github.com/langchain-ai/langchain/actions/runs/12180395579/job/33974633950),
but added a test to langchain-chroma as well.
