Fix checkbox defaultChecked state interpreted as not provided

[Host32]

AvCheckbox with default values was not correctly treating defaultChecked propertie, because a undefined value is treated as not provided, so the checkbocks is marked as checked even than the value is not present on array.

Reference:
https://stackoverflow.com/questions/55259173/react-handling-multiple-checkboxes

So just adding a corrent treating for undefined return of find call solved the problem.