community: Repair embeddings/llamacpp's embed_query method (#29935)

**Description:** As commented on the commit
[41b6a86](41b6a86bbe)
it introduced a bug for when we do an embedding request and the model
returns a non-nested list. Typically it's the case for model
**_nomic-embed-text_**.

- I added the unit test, and ran `make format`, `make lint` and `make
test` from the `community` package.
- No new dependency.

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/community/langchain_community/embeddings/llamacpp.py

@@ -139,7 +139,7 @@ class LlamaCppEmbeddings(BaseModel, Embeddings):
             Embeddings for the text.
         """
         embedding = self.client.embed(text)
-        if not isinstance(embedding, list):
-            return list(map(float, embedding))
-        else:
+        if embedding and isinstance(embedding, list) and isinstance(embedding[0], list):
             return list(map(float, embedding[0]))
+        else:
+            return list(map(float, embedding))

libs/community/tests/unit_tests/embeddings/test_llamacpp.py

@@ -38,3 +38,11 @@ def test_embed_query(mock_llama_client: MagicMock) -> None:
     result = embeddings.embed_query("Sample query")
     expected = [0.1, 0.2, 0.3]
     assert result == expected
+
+
+def test_embed_query_not_nested(mock_llama_client: MagicMock) -> None:
+    mock_llama_client.embed.return_value = [0.1, 0.2, 0.3]
+    embeddings = LlamaCppEmbeddings(client=mock_llama_client)  # type: ignore[call-arg]
+    result = embeddings.embed_query("Sample query")
+    expected = [0.1, 0.2, 0.3]
+    assert result == expected