I made the field optional and selected "not set" as the default, but it didn't work, still the field was rendered with option "No" selected.
However, I was able to remove the default value by specifying a Macro ({% return null; %}).
Now both radio buttons appear un-checked.
However, now I am having issues triggering the validation rules.
The field seems to skip all the validation rules. I tried regular expression, email, general condition validation rules.
Anyway I will keep trying from my end.
Thanks for your help.