box/console: Test for nil value in args directly

Instead of allocating a variable for optional args
testing we should use dot notation instead. Otherwise
it won't work for trivial test case as

```
require('console').set_default_output('lua,block')
require('decimal').new('1234.5678')
```

 | builtin/box/console.lua:47: expected decimal, number or string as 2 argument

and program exits.

Part-of #3834