bugfix: Handle prompts with no Chat messsages (#3892)

Also require API key  for langsmith

src/backend/base/langflow/components/prompts/LangChainHubPrompt.py

@@ -5,6 +5,7 @@ from langflow.inputs import StrInput, SecretStrInput, DefaultPromptField
 from langflow.io import Output
 from langflow.schema.message import Message
 
+from langchain_core.prompts import HumanMessagePromptTemplate
 
 import re
 
@@ -40,9 +41,15 @@ class LangChainHubPromptComponent(Component):
         if field_name == "langchain_hub_prompt":
             template = self._fetch_langchain_hub_template()
 
+            # Get the template's messages
+            if hasattr(template, "messages"):
+                template_messages = template.messages
+            else:
+                template_messages = [HumanMessagePromptTemplate(prompt=template)]
+
             # Extract the messages from the prompt data
             prompt_template = []
-            for message_data in template.messages:
+            for message_data in template_messages:
                 prompt_template.append(message_data.prompt)
 
             # Regular expression to find all instances of {<string>}
@@ -103,6 +110,10 @@ class LangChainHubPromptComponent(Component):
     def _fetch_langchain_hub_template(self):
         import langchain.hub
 
+        # Check if the api key is provided
+        if not self.langchain_api_key:
+            raise ValueError("Please provide a LangChain API Key")
+
         # Pull the prompt from LangChain Hub
         prompt_data = langchain.hub.pull(self.langchain_hub_prompt, api_key=self.langchain_api_key)