qdrant[patch]: Make path optional in from_existing_collection() (#21875)

## Description

The `path` param is used to specify the local persistence directory,
which isn't required if using Qdrant server.

This is a breaking but necessary change.

libs/partners/qdrant/langchain_qdrant/vectorstores.py

@@ -1353,8 +1353,8 @@ class Qdrant(VectorStore):
     def from_existing_collection(
         cls: Type[Qdrant],
         embedding: Embeddings,
-        path: str,
-        collection_name: str,
+        path: Optional[str] = None,
+        collection_name: Optional[str] = None,
         location: Optional[str] = None,
         url: Optional[str] = None,
         port: Optional[int] = 6333,
@@ -1376,6 +1376,10 @@ class Qdrant(VectorStore):
         This method will return the instance of the store without inserting any new
         embeddings
         """
+
+        if collection_name is None:
+            raise ValueError("Must specify collection_name. Received None.")
+
         client, async_client = cls._generate_clients(
             location=location,
             url=url,