fix(lua): vim.tbl_get({}, nil, 1) should return nil #32218

Problem:
`vim.tbl_get(tbl, nil, 1)` returns `tbl` itself. In this case, `keys` is not
empty, but `ipairs` skips the iteration:

    local keys = { nil, 1 }
    assert(#keys == 2)
    for i, k in ipairs(keys) do
      assert(false, 'unreachable')
    end

Solution:
Use `select("#", ...)` and `select(i, ...)` to ensure consistency for count and
iteration.

runtime/lua/vim/shared.lua

@@ -526,15 +526,15 @@ end
 ---@param ... any Optional keys (0 or more, variadic) via which to index the table
 ---@return any # Nested value indexed by key (if it exists), else nil
 function vim.tbl_get(o, ...)
-  local keys = { ... }
-  if #keys == 0 then
+  local nargs = select('#', ...)
+  if nargs == 0 then
     return nil
   end
-  for i, k in ipairs(keys) do
-    o = o[k] --- @type any
+  for i = 1, nargs do
+    o = o[select(i, ...)] --- @type any
     if o == nil then
       return nil
-    elseif type(o) ~= 'table' and next(keys, i) then
+    elseif type(o) ~= 'table' and i ~= nargs then
       return nil
     end
   end

test/functional/lua/vim_spec.lua

@@ -970,6 +970,7 @@ describe('lua stdlib', function()
     )
     eq(NIL, exec_lua("return vim.tbl_get({}, 'missing_key')"))
     eq(NIL, exec_lua('return vim.tbl_get({})'))
+    eq(NIL, exec_lua("return vim.tbl_get({}, nil, 'key')"))
     eq(1, exec_lua("return select('#', vim.tbl_get({}))"))
     eq(1, exec_lua("return select('#', vim.tbl_get({ nested = {} }, 'nested', 'missing_key'))"))
   end)