Test experimental package with langchain on master branch. (#9621)

It's possible that langchain-experimental works fine with the latest
*published* langchain, but is broken with the langchain on `master`.
Unfortunately, you can see this is currently the case — this is why this
PR also includes a minor fix for the `langchain` package itself.

We want to catch situations like that *before* releasing a new
langchain, hence this test.

libs/langchain/langchain/llms/base.py

@@ -528,9 +528,13 @@ class BaseLLM(BaseLanguageModel[str], ABC):
                 f" argument of type {type(prompts)}."
             )
         # Create callback managers
-        if isinstance(callbacks, list) and (
-            isinstance(callbacks[0], (list, BaseCallbackManager))
-            or callbacks[0] is None
+        if (
+            isinstance(callbacks, list)
+            and callbacks
+            and (
+                isinstance(callbacks[0], (list, BaseCallbackManager))
+                or callbacks[0] is None
+            )
         ):
             # We've received a list of callbacks args to apply to each input
             assert len(callbacks) == len(prompts)