fix(pack): do not assert non-nil stderr #40544

Problem: On some systems `stderr` can be disabled. This results in not
  usable `vim.pack` since it asserted `stderr` to be non-nil.

Solution: Stop asserting non-nil `stderr`. The downside is that
  potential errors are not shown, but this is intentional since `stderr`
  is disabled on system level.

  Still assert non-nil `stdout` as its output is important for
  `vim.pack` to actually do its job. Disabled `stdout` is not something
  that can work with `vim.pack`.

runtime/lua/vim/pack.lua

@@ -276,16 +276,16 @@ local function git_cmd(cmd, cwd)
   local sys_opts = { cwd = cwd, text = true, env = env, clear_env = true }
   local out = async.await(3, vim.system, cmd, sys_opts) --- @type vim.SystemCompleted
   async.await(1, vim.schedule)
+  local stderr = vim.nonnil(out.stderr, '')
   if out.code ~= 0 then
-    error(out.stderr)
+    error(stderr)
   end
-  local stdout, stderr = assert(out.stdout), assert(out.stderr)
   if stderr ~= '' then
     vim.schedule(function()
       vim.notify(stderr:gsub('\n+$', ''), vim.log.levels.WARN)
     end)
   end
-  return (stdout:gsub('\n+$', ''))
+  return (assert(out.stdout):gsub('\n+$', ''))
 end
 
 local function parse_semver(x)