detect all types of null default value

[pngwn]

ALternative to #1645.

• This PR checks for all types of 'null' default value (i.e. the value we get when no default value is provided), fixing the interactive/ non-interactive autodetection when no default value is given.
• I had to modify the preprocessing for Radio (which Dropdown inherits) as it was selecting the first value for the user even when none was given. I can kind of understand the reasoning but if we do this we have no way of knowing whether or a not a default value is being passed or not. I don't think we should be selecting a value for the user if they have not provided a valid one.

Closes #1569.

[aliabid94]

makes sense to me