fix: gemini content parts is nil (#1942)

lua/avante/providers/gemini.lua

@@ -137,6 +137,7 @@ function M:parse_response(ctx, data_stream, _, opts)
     local candidate = json.candidates[1]
     ---@type AvanteLLMToolUse[]
     local tool_use_list = {}
+    if candidate.content.parts ~= nil then
       for _, part in ipairs(candidate.content.parts) do
         if part.text then
           if opts.on_chunk then opts.on_chunk(part.text) end
@@ -153,6 +154,7 @@ function M:parse_response(ctx, data_stream, _, opts)
           OpenAI:add_tool_use_message(tool_use, "generated", opts)
         end
       end
+    end
     if candidate.finishReason and candidate.finishReason == "STOP" then
       OpenAI:finish_pending_messages(ctx, opts)
       if #tool_use_list > 0 then