community: fix neo4j from_existing_graph (#24912)

Fixes Neo4JVector.from_existing_graph integration with huggingface

Previously threw an error with existing databases, because
from_existing_graph query returns empty list of new nodes, which are
then passed to embedding function, and huggingface errors with empty
list.

Fixes [24401](https://github.com/langchain-ai/langchain/issues/24401)

---------

Co-authored-by: Jeff Katzy <jeffreyerickatz@gmail.com>

libs/community/langchain_community/vectorstores/neo4j_vector.py

@@ -1449,6 +1449,8 @@ class Neo4jVector(VectorStore):
                 "LIMIT 1000"
             )
             data = store.query(fetch_query, params={"props": text_node_properties})
+            if not data:
+                break
             text_embeddings = embedding.embed_documents([el["text"] for el in data])
 
             params = {