fix: move opts cast after second tbl_deep_extend in render_buffer_async

Problem: the ---@cast opts -nil was placed after the first opts guard
but LuaLS loses narrowing at the second tbl_deep_extend on line 928,
causing a persistent need-check-nil warning at opts.refetch.

Solution: remove the redundant first opts = opts or {} guard (the
tbl_deep_extend already handles nil) and place the cast after the
second tbl_deep_extend where opts is actually used.

lua/oil/view.lua

@@ -911,8 +911,6 @@ local pending_renders = {}
 ---    refetch nil|boolean Defaults to true
 ---@param caller_callback nil|fun(err: nil|string)
 M.render_buffer_async = function(bufnr, opts, caller_callback)
-  opts = opts or {}
-  ---@cast opts -nil
   local function callback(err)
     if not err then
       vim.api.nvim_exec_autocmds(
@@ -928,6 +926,7 @@ M.render_buffer_async = function(bufnr, opts, caller_callback)
   opts = vim.tbl_deep_extend("keep", opts or {}, {
     refetch = true,
   })
+  ---@cast opts -nil
   if bufnr == 0 then
     bufnr = vim.api.nvim_get_current_buf()
   end