allow deepseek spec to print out the last message with final stats

lua/kznllm/specs/deepseek.lua

@@ -53,7 +53,7 @@ local function handle_data(line)
 
   if data and data:match '"delta":' then
     local json = vim.json.decode(data)
-    if json.choices and json.choices[1] and json.choices[1].delta and json.choices[1].delta.content then
+    if json.choices and json.choices[1] and json.choices[1].delta and json.choices[1].delta.content and json.choices[1].delta.content ~= '' then
       content = json.choices[1].delta.content
     else
       vim.print(data)