Updated documentation about currentCode null vs empty

[cybex-dev]

Follows using setState({}) inside onCodeChanged resulting in build error if currentCode is null rather than just empty.

Code in question: https://github.com/jaumard/sms_autofill/blob/master/lib/sms_autofill.dart#L119

[jaumard]

Hey maybe instead of adding a note what we can do it wrap that call into an addPostFrameCallback, that will prevent any crash. What do you think?