chore(clipboard): accept nil (#371)

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

lua/avante/clipboard.lua

@@ -39,8 +39,9 @@ M.setup = function()
   end
 end
 
----@param line string
+---@param line? string
 M.paste_image = function(line)
+  line = line or nil
   if not Config.support_paste_image() then
     return false
   end