Refactor llm initialization

llama-index-core/llama_index/core/chat_engine/context.py

@@ -73,7 +73,7 @@ def from_defaults(
         **kwargs: Any,
     ) -> "ContextChatEngine":
         """Initialize a ContextChatEngine from default parameters."""
-        llm = llm_from_settings_or_context(Settings, service_context)
+        llm = llm or llm_from_settings_or_context(Settings, service_context)
 
         chat_history = chat_history or []
         memory = memory or ChatMemoryBuffer.from_defaults(