fix: resolve pre-existing LuaLS typecheck warnings

Problem: CI typecheck fails with 13 warnings — 11 need-check-nil in
util.render_text (opts param annotated nil|table but guaranteed non-nil
after tbl_deep_extend), 1 undefined-doc-param in view.render_buffer_async
(annotation says "callback" but param is "caller_callback"), and 1
need-check-nil on opts in the same function.

Solution: add ---@cast opts -nil after the tbl_deep_extend call, fix
the param name in the doc annotation, and add opts = opts or {} guard.

lua/oil/util.lua

@@ -582,6 +582,7 @@ M.render_text = function(bufnr, text, opts)
     h_align = "center",
     v_align = "center",
   })
+  ---@cast opts -nil
   if not vim.api.nvim_buf_is_valid(bufnr) then
     return
   end

lua/oil/view.lua

@@ -909,8 +909,9 @@ local pending_renders = {}
 ---@param bufnr integer
 ---@param opts nil|table
 ---    refetch nil|boolean Defaults to true
----@param callback nil|fun(err: nil|string)
+---@param caller_callback nil|fun(err: nil|string)
 M.render_buffer_async = function(bufnr, opts, caller_callback)
+  opts = opts or {}
   local function callback(err)
     if not err then
       vim.api.nvim_exec_autocmds(