fix(LiteLlm): add fallback user message to prevent empty content error with include_contents='none'

src/google/adk/models/lite_llm.py

@@ -824,6 +824,19 @@ async def generate_content_async(
         _get_completion_inputs(llm_request)
     )
 
+    if not messages:
+      messages = [
+          ChatCompletionUserMessage(
+              role="user",
+              content=(
+                  llm_request.config.system_instruction
+                  if llm_request.config.system_instruction
+                  else "Handle the requests as specified in the System Instruction."
+              )
+          )
+      ]
+
+
     if "functions" in self._additional_args:
       # LiteLLM does not support both tools and functions together.
       tools = None