Issue with printing arguments for should_be_called_with_any_args

```lua
f:should_be_called_with_any_arguments():
  and_then(f:should_be_called_with_any_arguments()):
  when(function()
    f(1)
  end)
```

Something like this will trigger it. It manifests as an error in https://github.com/ryanplusplus/mach.lua/blob/master/src/mach/format_arguments.lua because `args` is `nil`.