fix(lua): always return nil values in vim.tbl_get when no results

While `return` and `return nil` are for most intents and purposes
identical, there are situations where they're not. For example,
calculating the amount of values via the `select()` function will yield
varying results:

```lua
local function nothing() return     end
local function null()    return nil end

select('#', nothing()) -- 0
select('#', null())    -- 1
```

`vim.tbl_get` currently returns both nil and no results, which makes it
unreliable to use in certain situations without manually accounting for
these discrepancies.

test/functional/lua/vim_spec.lua

@@ -512,6 +512,8 @@ describe('lua stdlib', function()
     eq(NIL, exec_lua("return vim.tbl_get({ unindexable = function () end }, 'unindexable', 'missing_key')"))
     eq(NIL, exec_lua("return vim.tbl_get({}, 'missing_key')"))
     eq(NIL, exec_lua("return vim.tbl_get({})"))
+    eq(1, exec_lua("return select('#', vim.tbl_get({}))"))
+    eq(1, exec_lua("return select('#', vim.tbl_get({ nested = {} }, 'nested', 'missing_key'))"))
   end)
 
   it('vim.tbl_extend', function()