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.

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)